few questions:
if i'm getting grabbed during shield pressure, is it because i'm jumping too late out of shine? or is it an ariel timing thing? or both?
also, when peach crouch cancels, do i have time to hit with and a nair and still shine?
It depends where you get grabbed.
shine has 8 frames of shield stun. most grabs come out frame 7.
jump out of shine as early as frame 4, leave the ground on frame 4 (or 3?), nair comes out on frame 4.
if you aerial -> shine -> immediate aerial, it is hard to grab, but even if you pull back really hard after you do your second aerial, you will still probably get grabbed, depending on whether they mess up or are playing characters with no grab range
if you want to delay your second aerial (maybe they have grabbed you after your second aerial 5 times this match already), then you can do that too. this will either keep them in shield stun or hit them (if they are playing the same way as they did before).
best (least risk) thing to do is to cross them up or just shine grab. the best (best thing to do) is to start with safe options and then become unpredictable whenever you hit their shield
edit: i think for fox too (as well as falco), if your nair is stale then they can grab you before shine comes out after the first aerial. the timing is all pretty tight though