Depends on the motor. Most of my planes use an x-mount. Those screw into the firewall. Others, like the Emax 2012s, can use a stick mount or a machined aluminum mount that holds them in place with set screws. I have had the least luck with the latter.
All of the above. Plus hot glue when I am using speed 400-480s.
It depends on what plane it is, what motor it is, and what I have on hand.
I used zipties on a floaterjet motor for a Sea Blu baby. Why? because I ran out of ply.
Other than that I generally use screws for strength.
Screw it! Though I dont use ply, I tend to go with plastic. ABS is my friend it can take a hit and still be ok. I've also got some CF sheet that I've had for about 10 years now. I've had ply crack and break on bad landings, plus the tools I have at home, plastic is easier for me to cut.
Though I do end up makeing a lot of custom mounts for various planes. Not the bulk head but the mount that bolts to the motor. Working at a high end car stereo shop we have some metal fabing stuff at work. I grab some of the really small scraps of aluminum and will cut out my own mounts at home ith little more then a small drill press and a dremell.