I’ll take a stab at these questions!
1. Lets see, z axis steppers (there are 2 of them) each use 3 screws, x axis uses 3 and y axis uses 3 so I believe you are correct, only 12 needed. I have stripped a few so it definitely doesn’t hurt to have a few extra.
2. I was skeptical about the zip ties to keep tension on the belts but it works really well. I have no plans to change it. Also, I’ve ran my printer for many many hours and the belts haven’t lost any tension so far.
3. I don’t have my belts very tight at all. Just enough so that you don’t have any slippage on the stepper.
4. I use a laptop right now, but I’m planning on moving to the SD card. One night I had a print fail because the computer rebooted for windows patches. Also, for long running prints, I don’t want my laptop on the entire time.
5. I was using Ponterface for troubleshooting/testing in the beginning and then moved onto Repetier-Host. Now I’ve moved on to MatterControl. I really like the interface and it feels like the printer is running better than it did on Repetier. I hear great things about Simplify3d but it’s an extra $100 or so cost for that one.