I don't think it has changed, but I think it's more than just one level (maybe 2?). I know you don't get rested for turning in quests, so if you're getting a bunch of xp from quest turn ins that can push your rested bar beyond just 1 level. I've had rested get pushed far enough that when I gained a level, I showed a full bar of rested xp going into the next level.
No sir. It's just one. If you leave an alt parked on fleet for about a week and mouse over his experience bar it'll say something like "12,345/678,000 experience (678,000 rested)"
It never goes beyond the total needed for the current level, not counting how far along you already are.
So whether you are at 1/678,000 or 677,999/678,000 it will always top out at 678,000.
But you are correct that if questing, you can push that rested xp for two or three levels before the bar turns yellow again.