Kinoliker answer to question 1;
optical vignetting is basically the loss of speed, (not mechanical vignetting like dark corners), there is no upper limit, but once you go past 38mm you start to loose speed.
answer 2: a lack of squeeze doesnt exist, however a lack of overcompression exists (more squeeze on the corners), by using either a too short focal length, or a too long lens (like anon pancake design), or placing the scope too far away.
answer 3 if you do it right a magnetic adapter wont add much depth to your build however it will a tiny bit, so on the widest angles it might occur that you wil have more vignetting. simply the same rulle applies.. if your lens has a too wide angle of view, or the scope is placed too far away from the focal point, you will get vignetting
