I must say, this is a great thread. There are a few things I would like to chime in on:
Yes, you couldn't see an extremely long range laser strike coming in unless you had FTL sensors. You also couldn't aim an extremely long range laser strike without FTL sensors. Also, at some point, non-naval class lasers would still attenuate under a dangerous energy level at a certain range. In a society that has trouble feeding and watering its people, I don't see too many lasers getting very well tuned.
Yes, you could build a heat-seeking missile, but you couldn't fit the IFF computers into the warhead without losing enough power to damage Mech grade armor. Again, they have trouble feeding and watering their people.
Yes, one armor would stop energy, kinetic, electric, and explosive munitions. That armor would be a multi-layer composite with the various layers being more effective at stopping specific types of armor. This should sound fairly reasonable today, since How Its Made and various Military channel shows have examined the M1A1 Abrams armor, which is good example of this kind of armor tech.
As for the weapons and computer sizes, remember, this is after 3-4 apocalyptic wars depending on how you consider the 3rd Succession War. Even Star Trek had humans losing tech after one apocalypse. The other thing that many seem to over look is the sheer loss of life that the wars entailed. 52 Million died just in the Kentares Massacre. That is more then twice the number of casualties in WWII.
Edit: sorry, I hate spelling in the wee hours of the morning.