The thing with traders that stop playing caravan is intentional and was added in the last patch. Probably to stop people making absolute tonnes of money from them.
I have also experienced Caravan refusal on the part of the traders, which began with Lacey at Mojave Outpost. At first I kind of laughed, given that I'm 47/1 or 48/1 (could be 50, wasn't paying close enough attention to it) & have won tens of thousands of caps with Caravan. I figured she had had enough of my superior abilities, so (as also Death Reaper posted) I went to see Cliff Briscoe in Novac, where I learned that he also refuses Caravan. To my chagrin, Johnson Nash in Primm also blew me off. Rather unfortunate, I thought, given that I have 2 caps in my pocket and was given no warning whatsoever of such a "house-limit" which applies to the game in general. Can someone please verify Caravan restrictions having been included in the last patch? Lacey keeps greeting me with "That last hand, that's just my luck" so it seems like a bug from that angle, although its absence from the dialog tree in general makes it seem like a game-wide limit restriction. And if it is a game-wide restriction, I wish it had been handled a bit more deftly...like maybe giving the trader AIs more intelligence in playing the game in first place? None of the traders ever played a Jack against me, which is the surest way to keep the opponent at a disadvantage in the game. The issue could have been handled from several different angles, rather than simply evaporating the option from the dialog tree, leaving the player to wonder if it is simply another one of the glitches to be endured in the game.
Regarding the triple 26s, I've seen that happen a couple of times. Even if one of the contracts is tied at 26, shouldn't the win then be decided by the other two contracts? I've had triple 26s against an opponent without a tie and had to discard (looking for a Jack) until the deck ran out before the game was decided, but that doesn't seem right.