Yes, the twisted stretcher support is precisely the problem.
Your proposed 33 x 28 inch portrait seems right on the line to me. You may be able to use standard stretcher strips, but personally I'd cross-brace the longer dimension. You're rarely sorry to have built something too strong.
So as not to get too far afield from the thread's topic, I won't get into building frames, but I find it a lot of fun to creatively puzzle together various moldings into a custom frame. Perhaps another thread, another time. Meanwhile, this type of built-up frame is nicely illustrated in
How To Make Your Own Picture Frames, by Hal Rogers and Ed Reinhardt.
Lastly, on the odd-length stretchers. That's exactly what I ran into on the landscape (a problem subsequently mooted by gravity). I bought a pair of stretcher strips 2 inches longer than needed, then somewhat painstakingly used a Japanese handsaw to cut new French miters into them at the custom length. Interesting once, but I don't wish to make a habit of it.