It's difficult to diagnose remotely but ... it looks like some multiple factors got you. From your other thread, you may need to address the trailing edge bend. This is not easy to redress once the structure is in because the structure itself interferes with further bending. Another factor is that it looks like the flanges of the tip rib are not 90 degrees to the web. When fluting, you have to make sure the web lays flat and the flanges remain at the proper angle. Yet another factor might be the attachment of the tip rib to the spar; I think the ribs are symmetrical but could you have canted the rib slightly ... maybe when drilling and installing the elevator horn? That last is about the only thing I can think of that would really cause your problem; otherwise I'd expect the skin to twist the skeleton to fit.
For corrective action, I'd say carefully drill out the inboard tip rib and enough spar to allow you to remove the horn and rib. At this point, see if the skin goes back to a straight trailing edge. If not, you'll need to work the skin to get it straight again. While keeping it straight, reinstall the rib (I'm going to suggest a replacement rib because I think you're going to have to rework it so the holes won't line up anymore; ask for an unpunched rib). Make sure the flanges are 90 degrees and don't push it so far back to the trailing edge that it makes the skin bulge as in one of your pictures. If necessary, remove the flange for attachment at the spar and replace with a bit of angle to allow the rib to be located forward (the strength at that joint is in the elevator horn anyway. Once the rib is in with the skin straight, then drill the horn, being careful not to twist the elevator. The weldments aren't exact due to the welding process anyway and some allowance is made when aligning the elevators later so don't worry if the best fit of the weldment is not perfectly centered in the rib. If I am right about the cause of the problem, that should take care of it with minimal effort and cost.
Edit: I noticed that the bulge at the rib I mentioned is at the outboard tip (6th picture), though I'd still look for it at the inboard rib, too. For the outboard rib, short of reworking the rib and counterweight, you could try enlisting the help of a body man to work the bulge down. It's delicate work, though, and the bulge is not that bad as long as there is no twist there, so you could just leave it and forget it. I've seen worse ... on my own project. Building my -6A was a learning experience that I'm now applying to the -10. But the -6A flies great even if it is a little rough.
