this is pure speculation since we never got to know how competent she was. just because she was young & fit-looking doesn't necessarily mean she can breeze through the roadblocks.
Not really. She did complete the "hardest Roadblock ever" (according to Phil). It'd be speculation to assume she could definitely beat Karen in a head-to-head climb or that Colin/Christie would necessarily correctly divide the Roadblocks, but it's a fact Christie could have done half the Roadblocks with little difference to the team placements:

Leg 3- chocolate-eating. Luck-based and since they were first (or 2nd?) they had good odds. Even if she sucked at it, Jim/Marsha were far behind enough they wouldn't be eliminated
Leg 5- going down a hole. No difference in their placing (though they did the FF, so they could skip it anyway).
Leg 6 - digging for scarabs. Luck-based and even if she gets unlucky, Marshall/Lance quit
Leg 9 - brickmaking. Colin sucked at it. Even if Christie failed, she would have a huge time buffer bc of the FF. And it was an NEL even if she completely bombed it.
Leg 10 - zorbing. Which would depend on whether or not Colin would let her do it, but she couldn't mess it up.
Leg 11 - climb the bridge. Everyone got bunched up and they ended up in last place anyway, so their performance made no difference.

That's more than half where she could either couldn't fail (e.g. zorb/climbing) or could fuck up with impunity (e.g. brickmaking, scarabs).

The only situation where I could potentially see Christie doing a Roadblock be so castastrophic that it could cause their elimination would be the ostrich egg - and Colin said on the show that he wouldn't make her do another eating challenge after the trauma of the caviar.