1) Fusion 360 for CAD/CAM.
For Machine control, either LinuxCNC, if you don't mind using Linux, or one of the Arduino /GRBL options.
2) About 100 different ways. Some like T -Slots in the table. I use T-Nuts in the underside of my table. Some people just screw parts down to their spoilboard.
3) Depends on what tools you have. I'm not real familiar with the parts, so can't really answer your question. Forstner bit, or maybe a hole say will work as well. A drill press is very helpful, especially for forstner vits.
4) A torsion box is a very rigid box structure.
5) not sure.
6) A 2 start screw has 2 threads, that alternate. So, for each turn, it's moving twice as far (and twice as fast). There are cheap 5 start screws on Ebay.
7) Whatever is cheaper.
8) Try Ebay. For smaller pieces, it won't split apart, so it's quite a bit stronger.
9) You didn't ask a question. You should be able to figure it out as you go. Spend some time reading through all the build logs here. They should help.
10) You'd probably be better off with something like these: US Ship Stepper Motor Driver 1.5A-4.5A 24-50VDC for Nema 23/Nema 34 Motor M542T | eBay
You want somewhere in the 280-380oz range.
Don't buy any electronics until you have the machine built, and really understand what you need.
Lovejoys are fine.
Voltage is 32 times square root. 32 x 1.8 = 57V. You really want somewhere between 36V-48V. Speed is proportional to voltage, so the higher the voltage, the faster the machine will go.