You haven’t calculated 1/120 x 1/120, etc. You have calculated 1/(8x120). You mean to calculate (1/120)^8, which gives 1/42,998,169,600,000,000.
You've misunderstood the correct calculation. P(t1) is the ENTIRE draw with a derby as the first tie drawn. P(t2) is the ENTIRE draw with a derby as the second tie drawn. They ARE separate events. There are eight possible ties where a derby could be drawn, and I’ve calculated the probability of each (not surprisingly it’s the same for each) and added them up. The first tie not being a derby is calculated first in each instance (for the seven instances where the derby is after the first tie drawn) as 14/16 x 13/15 (the total non-Sheffield teams remaining over the total number of balls remaining).
If the probability of us drawing Wednesday is 1/960, and given the probabilities of us drawing any other ball must be equal, you have a draw where there are apparently 960 possible outcomes for SUFC: one is Wednesday, 14 are the other balls and 945 are unaccounted for.
If you had calculated what you meant and the probability of us drawing Wednesday is 1/42,998,169,600,000,000 (this is hilarious) then you have 42,998,169,599,999,985 outcomes where we don’t have a fifth round tie.
You’ve had an absolute mare here.