1: Yes: https://docs.gimp.org/en/gimp-view-flip-rotate.html
2: The same that you would tilt a piece of paper when drawing. Not very
useful if you edit photos with your mouse, but a lot more if you pain
using a tablet.
3: Yes, just hit '!' (exclamation mark)
4: Not as far as I know. Note that in
GIMP 2.10.30 on Xubuntu 20.04
While performing other operations, I have found that the entire frame of
reference for gimp (image, guides, etc.) is rotated by holding Shift-B2
(middle mouse button) and dragging the mouse. This always happens
accidentally and I have found no way to reset the view
