1) 4 or 5 will be good enough.
2) Use whatever you can get that suits your budget. You won't be pushing for every last bit of performance, and the neglible amount of play in a belt isn't going to matter in the grand scheme of things.
3) It will cause racking, especially if you're gib isn't adjusted correctly, however you just compensate for it in either via tool offsets, or the part g-code.
4) Measuring is pointless unless you already have ballscrews on. My last lathe used a 1.47nm stepper directly coupled to a 12x4mm ballscrew on the Z-axis, and it could sink a 8mm drill into aluminium, but would only mark steel. My current lathe has a 750w servo connected at some stupid ratio (about 1.3:1 IIRC) to a 16x5mm ballscrew, and it'll knock the turret out of alignment before it'll stall the servo!
Personally, I'd say go for some of the good quality high torque NEMA 34 steppers combined with good drives and see how you get on with a 2:1 reduction. You can always change ratio later.
5)On a lathe, backlash isn't really an issue, as you're nearly always cutting from a single direction with each tool, so you can compensate pretty well. It's not like a mill, where backlash can cause havoc when trying to cut with the cutter digging in and moving in the backlash. Do some searching on ballscrews. Generally you get them already assembled and from the same supplier.
6) QCTP, Gangtooling, or a combination of both are the cheaper options.
7) Whatever. They're all a much of a muchness in terms of performance on a lathe. It all comes down to how much you want to spend.
8) That will depend on what EMC will accept. I use Mach3 and my old lathe only had a single pulse opto slot sensor connected to a smoothstepper, and it threaded perfectly well even though it only had a 750w DC motor.