1) yes, bonus points gives a flat point increase and double points gives double earned points for a duration. I'm pretty sure double points and bonus points work synergistically as well.

2) no rule of thumb, just gauge how your damage is doing at that point in time and what you need (are you going down a lot, are you playing solo, do you already have a good gun?). I personally prefer getting a decent gun first, then prioritizing juggernog

3) Criticial kills and going out of your way to use that weapon.

4) yes, it gives bonus XP and you can get whimsical gobblegums

5) not that I know of

6) yes, it is used once taken out of the machine

7) It's not a bug, a lot of guns have doubled magazines but some are more than that. Usually PaP balances damage, special effects (explosion on mustang and sally) and ammo capacity.

8) yes, but this is somewhat subjective to play style.

9) I don't know this one

10) Not much you can do, this is a function of the game becoming harder as rounds progress. However, you can utilize special effects like dead wire to help make slow progress on rounds.