I've almost all but decided to purchase a 45mm TS-E lens to do landscape work with. I haven't found the answer to a couple of questions and I'm hoping someone might be able to help?
How much resolution gain can be expected from a full shift stitch with the camera in a vertical position on a 5D MKII?
In millimetres, you'll add 2x 11mm to the 24mm in portrait orientation, so your virtual sensor becomes 46x36mm, which translates to 7176 x 5616 pixels (40.3 MP) on the 5D Mark II. Of course the image quality at the fully shifted edges will be a bit less than in the center of the image circle.
Can a stitch be made without needing the camera to be level to the horizon?
Yes, by shifting you only increase the image circle you can capture (in 2 or 3 shots), and thus increase the field of view. The aspect ratio does become a bit more square (2:2.56 instead of 2:3), so you may want to crop for the best composition. That's IMHO one of the benefits of rotational stitching instead of shifting, one can add as many tiles as the subject
requires rather than letting the image size
dictate the composition.
I ask that last question because when I had a RRS Pano stitching kit it always had to be level to the horizon.
It's a pitty that you don't seem to have that kit anymore, because only shifting the lens (entrance pupil) will introduce parallax. With as small addition of 2 stopbars and a sliding clamp set you could have solved that (a shorter bar can be used if size is a concern), as I have:
That works pretty fast, shift lens left and camera right, shift lens right and camera left, and there will be no parallax when the stop bars are positioned correctly (for which I made a simple spacer for setting up).