There are some raindrop guides on right walls, but how do you do it on left walls (i.e. like in the Hell Temple Hard Mode run)?

If you're talking about the room with the box puzzle, I recreated it by "grapple clipping", grappling to the wall right before landing on an object. Normally you use it with pots against left walls, and normally you zip left instead of entering the wall in a raindrop like state so I assume its unique to thicker walls like the Hell Temple's? Additionally it seems like the box has to be pressing on a switch for Lemeza to enter the wall, tried with box on switch in the room with the Lamp shop and got stuck in the wall, tried pushing a box not on a switch against a left wall in Inferno Cavern and Lemeza left the wall after jumping. I had no idea about this phenomenon until you pointed it out though so grain of salt etc.

Raindropping on left walls is not a thing. The clip in hell temple is probably a grapple clip. (consult the glitch companion found in resources) You'll get a better answer faster by asking in the discord, since there's proper nerds there.