Your adaptation of the 50-move rule for A.C. looks good. It's simple, logical, and is pretty much optimal in terms of its main purpose of limiting the duration of the game.

Regarding the marathon game, the main concern involved dual-King positions, so ending the game immediately in such situations should obliterate that problem entirely. It will also lead to a sharper, more volatile game, due to many strange cases where the action seems to just "end" suddenly in the middle of a major skirmish; not sure whether that's a good thing or a bad thing.

If the King is in the corner, then only 13 points of material are needed for checkmate (assuming you are referring to the points used in A.C., and that the friendly King, to avoid ambiguity in the point counting, doesn't participate). For instance, with Black King on f4, and White having "soldiers" on d3, e2, and f2 and an "Advisor" on d4, moving the Advisor to e3 results in checkmate. Of course, this doesn't really add anything relevant to the discussion, as the fact remains that checkmate is a highly unrealistic possibility in a two-Kings endgame.

It's not easy to even weigh the pros and cons of weakening the Prince's and King's powers, let alone to compare the various possibilities on how to do this. One thing I've noticed, though, is that taking away the Prince's diagonal move will leave the f-pawns unprotected in the starting position. Because the b and g-pawns are also undefended, this is going to make the opening phase extremely unbalanced. White will almost always open with  1. E f1-d3, and play will inevitably center around targeting the f-pawn (Black will try to counterattack in the exact same manner, at the first opportunity), and an early Nh6 will likely be forced from Black just about every time. And I'm pretty sure White can force the exchange of a Black Chariot for a White Elephant, in fairly short order.

The other two proposed Princes are rather similar. Either way, removing the "dual-Kings rule" should still result in some long, usually very drawish endings, because such situations will most likely occur in relatively simplified positions (in addition to the nature of promotion and demotion; late in the game the opponent's side of the river will often be considerably safer). Of course, weakening the royal pieces WOULD significantly lower the percentage of games decided by the point system (because it will be a lot harder for the Prince to make it across the river in one piece).

That's pretty much all the input I have to offer. It really just amounts to a few things that you may want to consider when trying to decide on one rule over another. At the end of the day, it comes down to subjective choices such as how many moves should the average game last, how often should the point system decide the game, or how well-balanced should the game be in each phase, and as a whole - these are all up to you.

By the way, I've accepted your invitation (and chose White since I played as Black in my first game), but have yet to move - the reason being that all this talk of rule changes left me unsure what version of A.C. I was actually playing. I'll make the 1st move once the rules have been finalized. The amount of time I can afford to spend on my G.C. games is very limited as well, so don't worry about moving slowly. Just don't expect my moves to necessarily come in any faster :p