I think some teams (Trotters, Cowboys) are able to acclimate to the higher attitudes better than others. I don't think you lose your ability to adjust your breathing (and oxygen intake) from living in higher attitudes simply because you no longer live in such an area. (it increases your lung capacity, which is why some world-class athletes train in high altitude locations in their respective sport.)
As to whether Jamie and Cara could or couldn't U-Turn the Trotters after being U-Turned, I don't see anything in the descriptions we've had about the double U-Turns that preclude it. The limitation is that one you have U-Turned another team, you can't do it again in the Race (because you no longer have the smaller photo of your team in your fanny pack); with the Double U-Turn, using it on another trailing team especially when you've been U-Turn makes perfect strategy sense. (And we know that the team photo is the controlling factor because those were used with the Yield, and losing that team photo prevented some teams in that era from Yielding another team.