I've also thought about a diagonal lift, but haven't worked out the details to see if it would actually work. The thing to remember it that the farther back the flight deck is the less vertical room you have between the edge of the lift and the inside curvature of the fantail, and it'd make for a tight squeeze.
Yes. I tried that approach and found the angle not friendly to the boxy shuttles, especially given the size of the opening for the elevator as seen from the flight deck shots.
The other thing is that in JTB and TIS is that we see the back wall behind the shuttle while in the hangar deck and it looks vertical, IMHO.