You can go into the menu to toggle vertical mouse movement in full game runs. In ILs, however, you would need to have it toggled on beforehand and keep it on throughout the entire run since entering the menu during an IL for any reason will invalidate your run.

As far as I know, no macros or third-party tools/scripts that affect gameplay are permitted.

To answer your first question, pausing mid-run is will invalidate individual level (IL) runs, but pausing during full-game runs is allowed.

Second, to get better information than the generic "practice makes perfect" type of answer, I would suggest joining the Console Doom Discord if you haven't already (linked on the left side of this speedrun.com page) and asking there; there'll likely be people willing to help you out in more depth there.