NorthLondonRef - all I can say is that if you'd been the AR you would possibly have kept City in the Champions League by making the right decision, but not necessarily for all the right reasons.
You said, in your OP, "there has to be two players either in-line or behind Sane" so if Ederson was on his line, that would be two players so Sane would be onside however Sane is offside because there is only one players between him and the goal". Apart from Ederson was at the other end, that is exactly why Sane was in an offside position. Of the two players needed to be nearer the goal line to keep an opponent onside, one may or may not be the keeper.
That's cheered me up a bit but I do wonder how long you've been doing this!