How would you use top row End and PgDn on 65% and 75%?
by putting them on the keyboard
different people bind their nav keys differently
Sure, then let's include every possible permutation of rows and nav keys (or any keys, for that matter) in every set. Who knows, maybe someone likes to bind their End key where F usually is and swap their PgDn key with Backspace. Heck, why even have kits that cover the
most commonly used layouts in the first place; just make all the keys blanks and call it a day.
You have to be realistic. Those two keys don't make sense in those positions (R1) on boards smaller than a CP / 96-key. More specifically:
65% — End or PgDn on the top row? I don't think so.
75% — The only option I can think of that could hypothetically work is Home on the F row and End on the number row, with PgUp, PgDn and Fn below that. But it's a real weird one; I've never seen anyone use that layout, and it would certainly fail the commonness test. Moreover, 75% keyboards with no separation/blocker between the F and number rows are becoming less and less popular. So that's another reason why it's a bad idea.
In summation, I stand by what I said previously: R1 End and PgDn should be moved to the numpad kit, as has been the case in most recent GMK sets that have chosen to split numpad keys out of the base kit.