Well the lack of ability to build new whitestars is based on 2 things:
1) AoG fluff. I've said this on numerous occasions but JMS SAID the AoG books were cannon.
2) In in the beginning or season 5 (I cant remember when exactly) one of Sheridans reasons for building the Victory is that 'sooner or later we are going to run out of whitestars' since they cant build any more without the Vorlons.
Incidentally this is one of the reason I hate the entire concept of the whitestar gunship, carrier, fighter, etc. I dont mind them building more in future, with time and research Im sure they could EVENTUALLY reverse engineer the Vorlon biotech to some extent (though to fully reproduce it would take quite a long time in terms of R&D I suspect even for the Minbari).
As for the Victory's energy absobing crystal hull armour its explained in 'A Call to Arms'