Everything I've learned has come from the /wiki and just playing - that's ultimately how you have to find these things out. I don't know the answer to all your questions, but here's what I've discovered so far (it may be inaccurate or change as the server evolves):
I could only put two on my rifle. I believe backstab is just shooting someone from behind, but I've not tested it.
No, I have an M16A2 and it is full auto.
You may only have one primary gun in the hotbar - unsure about secondary. I'm also not sure if an SMG is considered a primary or secondary - I've not made one yet.
Every weapon seems to have a high damage and low damage based on aim. I'm not sure if it is high/low or a damage gradient. For instance my estimated max damages are around: sniper:27, rifle with fmj:27, assault rifle:15, pistol:13, shotgun:35 (at melee range). Also remember that each gun has perks/weaknesses even within the same category (sniper/rifle/pistol/...)
I've never gotten an armor blueprint, but armor is rarely dropped from zombies (that may have changed to not at all). I have some, but rarely bother with it. A bite is a bite, and it deteriorates. It disappears when it is used up.
Weapon damages seems to be based on aim and number of zombies hit. Worse aim or multiple zombies deals less damage.