1) 60 minutes is optimal, but anything between 50 and 75 minutes is good.
2) what to do: buy 3 scrubs for 1k each, and have them play friendlies. They can't play in your PO matches, but they can play scrimmages
3) yes.
4) it's randomized, so even with 60 minutes there may be drops, and even with too many minutes there may be ups, but I'd put the cutoff at about 80 minutes.