for some reason i sometimes land the zips in tunnel crawl but majority of the time especially in actual runs i'll either just fall, or i'll get through first part but then get no zips on 2nd half of the tunnel

If you're running on PS the timing is extraordinarily tight. It's a strat that I never tried in runs because the timing is near frame perfect

I found it useful to just do a few practice zips on the slope leading to the lower part, since it helps you to get accustomed to the timing, but overall expect to die quite a bit there.

I remember reading someone suggest using R2 + L1 to zip right back up in case you miss the R2 (which might help you save a few fails), but I personally don't use it. It's a bit ballsy to use that strat on an Easy Mode run imo (but it's nice if you pull it off).

Hard Mode though, that's a whole different story.

It's not luck at all. How I do is while crawling i jump off and press swing very fast to zip forward, and if the zip doesn't work you have to spam swing up to not die. It's about reaction time, you have to have 3 buttons ready to be pressed in short amount of time. jump + zip, if not zip up, repeat