Borderlands – What happens to XP earned after reaching the level 50 cap when adding DLC that raises the cap

I can't say for sure, but I would guess it won't allow you to play.

Character information is most likely stored in the game save, which was probably altered somehow by the DLC, so I guess a game without the DLC won't be able to read it properly.

I would test it for a more "official" answer, but I don't own the game.

It is true that the Borderlands ending is one of the most disappointing endings that a game ever had. Quoting Paul Helquist, senior designer:

I think what's great about where we did leave the player is that we have pretty much infinite options of where we want to go next[...]
[...]The ending of the game, for better or worse, didn't paint us into any corners.

So what I presume is that they will dig deeper into the story around the Vault in Borderlands 2 (released somewhere between 04-2012 and 03-2013).

To give you a little bit satisfaction from the ending, consider this explanation I found on the web a while ago, that made the ending slightly more acceptable:

The world is named Pandora, implying that something can be opening from it, this being the Vault. The legend of pandora's box detailed that the greatest evils of mankind were locked away in it, and someone was foolish enough to open it. Once the evil was released only one slither of goodness was left within, and that was hope. So think of the Destroyer as evil in a physical form, sealed away in Pandora's box so to speak, and the character you are playing is the one final hope for the planet.

I do hope that the ending in Borderlands 2 will be a better one than its prequel's.