Questions 1 and 2 are related: if you calculate payback by counting
months where cumulative cash flow is less than zero, you can use this
formula to count negative values in a range:
=COUNTIF(A1:L1,"<0")
.... where months 1 - 12 are in A1:L1. You may need to extend this
range if your timeline requires it. Also, add 1 to this figure to
reflect the first positive month.

Does question 3 refer to first positive month number, or the first
positive cumulative cash flow?