Those aren't stupid questions. Unfortunately some people here are too conservative and early to bark at people for raising questions for which the answers only seem obvious to them.
1)The question of ERA versus NERA is a matter of design philosophies, and wargame analysis.
In absolute terms, neither ERA nor NERA are more effective from one another.
They both have a linear tradeoff of capabilities. It is a function of single-hit protection, i.e how effective would one type be against only 1 shot, and the total number of shots that armor can take.
In a very rough comparison, an ERA can interact with 1 projectile resulting in X penetration reduction. And a NERA armor would defend from 2 projectiles, with only X/2 penetration reduction at a time.
On average, they are mathematically equal. On the battlefield, certain scenarios will show the superiority of one over the other.
To better understand the situation, you must first understand that actually both western and eastern tanks use NERA of some form, as the key component of their armor. Only for a short time was ERA ever dominant over NERA, and that was at the time of tanks like Leopard 1, AMX-30, and M48/60, because proper composite armor did not exist yet with the quality needed to defeat HEAT or APFSDS.
Next comes the impact probability analysis.
If a certain area is considered likely to be hit multiple times in tight groupings, over the course of a single engagement, then NERA is the preferred solution, even though again the Soviet tanks used NERA as much as western tanks.
If a certain area is only likely to be hit once in a single engagement, then ERA is preferred.
This is why on most western AFVs, the first type of applique to appear for side protection was ERA, and only after certain advancements, it moved on to NxRA (will get into that later).
And finally is the system's longevity analysis. Or basically how long the tank is expected to survive in either case.
Soviet tanks were considered more disposable than NATO tanks. Although fiercely competing with the west to create the higher quality tank, part of the philosophy was that even an advanced tank won't survive for very long on the battlefield. Minutes at best.
Thus ERA, being more effective for a single hit, would basically double the number of shots required to take out the tank, in some of the more likely scenarios.
And when you double some capability, for seemingly no cost at all, that's something worth doing, and is no longer an incremental upgrade.
In the west, tanks were expected to be more survivable, hence for example the human loader that was more of a spare than an actual necessity for normal operation of the tank.
With a focus on higher overall longevity of the platform on the battlefield, the ERA would not be more than a minor addition over potent NERA. It would be a single use item in an environment in which designers believed a tank needs to be able to sustain many hits, even if only for the sake of recovery.
Plus, it would encourage a bad culture of crewmembers' false reliance on a single use item, perhaps not fully understanding the extent of the danger in such belief.
But wherever sufficient NERA was not possible, western philosophy did not exclude ERA at all, and you can see the Bradley for example entirely covered in ERA.
2)Depends who you're looking at.
USA - Wanted Trophy more than a decade ago but Raytheon lobbied hard enough to delay its acquisition until it can complete its own system, which it eventually never has.
Reallocation of funds was also time consuming. Army doesn't always get what it wants, and almost never on time, unless Congress is especially generous.
Rest of NATO - For three reasons mainly.
First, they are very much disconnected from their MIC and will more often try to subvert the MIC than help it, because of a perceived sense of security.
Second, rest of NATO are being led, not leading new technological trends. Their innovators are their MIC which they don't do nearly enough to support.
Third, the acquisition of arms in Europe is done with the intent of deterrence, not the actual usage of said equipment in combat. Hence why you can still see Leopard 2A4 as the main MBTs of certain countries.
I've explained a long while ago, in depth, the economical effects of an APS. One of the conclusions was that it is economically unviable to buy AFVs without APS, if the AFVs are to be used during their lifetimes at least once in a medium to high intensity combat scenario. Most combat today is hybrid warfare, which is medium intensity. So basically for most of the combat we see globally, an APS is a must.
It is only viable to buy a tank without an APS if the tank is not expected to see combat.
That is why countries like the US, Israel, Russia, Ukraine, and Turkey, are seen investing in APS. Even poor Syria does.
A Leopard 2A7V, and its ancestor the Leo 2, form a lineage of 40 years of service in Germany. At no point were they used in serious combat by the Bundeswehr. Only the VJTF is supposed to be deployed abroad and expected to see combat on short notice, which is why the VJTF tanks will receive an APS. It just didn't get much publicity.
Of course, there are some within NATO who see the importance of capability maintenance and building regardless of the probability of war, and are investing in APS as well. The Netherlands for example are probably going to be the first in Europe to use an APS, on their CV90.
3)Russian ATGMs are not really a good comparison. They simply were never really effective weapons. Only effective within a small range of conditions.
A proper GLATGM would be something like a Spike downsized to 120mm, but today it's hardly necessary. There are two main considerations to this - tools, and tactics.
Tools - a tank battalion never drives alone. It will have infantry support. Infantry on the battalion level will always have an AT element capable of launching ATGMs at standoff ranges, and their vehicles have ATGMs as well, to multiply the output. Other than that, available tools include artillery, that in the modern day use long range guided missiles (+20km range), guided rockets, and guided/unguided shells.
Tactics - when spotting a tank formation of any size, 6-8km away, other options are preferred. Ambush with short range engagement from prepared positions is ideal.
The next best alternative is actually calling artillery or aviation, because the effects of a sudden barrage are going to be far greater, as opposed to an ATGM volley that would have the core of the formation maneuver away and screening their maneuver, when they see the first missile flying.
The third best option would be to lob HE shells, not ATGMs, at enemy tanks too far away.
The reason is that HE can do a lot of damage to the optics, gun, stabilizers, and other external equipment that is key for the effective use of the tank. It could even outright disable tanks by hitting the tracks or the UFP close to the driver's hatch.
ATGMs pack an HE payload as well, but are far less versatile and substantially more expensive, to the point where it's worth asking whether allocating vital space for them inside the ammo rack is even worth sacrificing other ammo types. HE-MP is just too versatile to not want it in greater numbers.
4)Basically all current MBTs can take a hit from a 152mm howitzer. That is, the crew will live, but the tank will be disabled.
This question is perhaps irrelevant, because howitzers on the battlefield are used in direct mode only rarely and in emergencies.
BONUS: Today there is something called NxRA. It differs from NERA and is somewhat of a replacement to it, rather than a competitor.
Anyone can feel free to correct me if I'm wrong, but basically:
NERA - Non Energetic Reactive Armor. It does not produce any energy on its own. It relies on the energy produced by the projectile and impacts the projectile with proportionately produced energy.
It's reactive, but more often than not regarded as passive because of its lack of independently energetic components.
NxRA - Non eXplosive Reactive Armor. Much like ERA, and unlike NERA it produces its own energy. However, it's not the blast you'll see with an ERA. It's more tame.
And the results are an armor that is as survivable as the NERA, but quite substantially more effective per shot than it. Not as effective per shot as ERA or SLERA (self limiting ERA), but it's somewhere close.
Because of this, NxRA is considered more effective than the NERA and ERA, because its per-shot-effectiveness to survivability ratio, is higher than both of them.
You can even see that the NxRA is gaining traction, and is now armoring tanks like the Merkava 4 entirely (or almost entirely), is offered for advanced variants of the Leopard 2, and armors the UAE's Leclerc.
It's also used on a plethora of medium AFVs like the CV90, Ajax and ASCOD, etc.
It's just not going to replace ERA everywhere because of a not too good volume efficiency that could make certain vehicles too large.
It really was one of the dumbest arguments I've ever heard when rationalizing combat capability degradation programs.
Basically every army that saw actual combat, decided the potential risks posed by ERA to infantry are greatly outweighed by the risk reductions it offers.
When APS became operational, only then has this idea become a frequent talking point. But APS is far less dangerous than ERA because it neutralizes the projectile's warhead without initiating it.
All because Raytheon couldn't deal with their loss.
LAHAT is only a shitshow if you insist on analyzing its capabilities OUTSIDE of its historical background.
It was devised for the Merkava 2 tank, long before the Spike even had half the capabilities it has today.
At the time, you needed LoS to the target to fire off a Spike, while the LAHAT allowed you to fire it off without LoS.
Another point you've forgotten is that a helicopter is not required for remote designation. It can be done via infantry. In any event of invasion into Israel, the first line of troops will be border brigades, not equipped with tanks and heavy weapons, but with a great deal of observation and intelligence capabilities. They, and the spearhead units', have plenty of infantry they would allocate to target spotting and designation either for artillery, AF, and whatever. They can easily designate targets for MBTs or helicopters using LAHAT. And they themselves would have a low combat signature.
Caliber is of course a non-factor because of top attack, hence why Spike missiles (except for SR) always had a relatively weak warhead compared with contemporary designs, even other similar sized missiles developed by the same company.
Only today are LAHAT missiles irrelevant, hence their withdrawal from service a long time ago, and their marketing to non modern armies.
It's not very accurate. The Namer and Merkava 3 and 4 may have ERA in some places.
It's not confirmed but the Mark 4 has armor plates with the inscription "explosive", and the Mark 3 and Namer have armor modlues with box shapes, suspiciously ERA-like.
Besides, certain Nagmachon variants can still be seen with the old Blazer ERA.