I highly doubt the tank is adjustbale, and I'm not sure if Crossfire makes adjustables, but since I'm not sure I won't give you a positive answer.
The bushmaster or defiant comes standard with Semi, 3/6 shot burst, and Full auto (totaling 4 modes of fire).
I'd say approximately 700 shots with the stock reg.
Yes you get the screws, they come with the gun, however if by some chance you don't or you want an extra one or two in case you lose them, they are found at any local hardware store.
If you want a really loong drop forward that one should be fine, I think you need an asa adapter, or you could look into a warped sports (for a screw in), or a Kapp DZII and if it's a screw in get the asa adapter.