You might consider using a linear hammer arrangement like the vz. 58. In this arrangement, the hammer rides in the BCG rather than the receiver, and catches on a spring-loaded element in the receiver. Where it differs from a Mauser-esque striker arrangement is that it is a separate piece from the firing pin, and impacts it like a traditional hammer.
With this arrangement, you could have the hammer cock and make up the distance between the back of the BCG to the FCG. Then the hammer collapses into the BCG minimizing its length and therefore the length the receiver has to be to contain the action's stroke.
Linear hammers do have some disadvantages - notably they load in the forward stroke using energy from the recoil spring, rather than on the rearward stroke using propellant energy. This means if you're not careful about your spring weights and mass ratios you could end up having trouble with failures of the bolt to close. The vz. 58 for example sometimes experiences this.
As for firing pin retention (your actual question!) maybe try what Glocks have on their slide and use a sliding plate?