I'll clear up a few things here, but I'm sorry you're having these issues.
I would like to mention that these spider levels have been designed to be difficult.
The pathfinding is not quote on quote
broken, the player is usually breaking it. For example, the spider is glitching as I am currently just pressing down on the underground and the spider can't correct it by going there. (Sorry if that sounded weird) The devs should know what I mean.
Edit here! —This works for me, don't right-click to move, that breaks it as well.
You don't need to hold anything down, just simply
LEFT click and the spider will correctly move. Example. [Enjoy these bad drawings] (I edited this reply to show some examples.)
Edit in picture: made slightly easier to read.
Also, as stated, the enemies want to go to the presents. Use the spider abilities to slow them down, it's the main purpose of “Crippling Venom“ which helps a lot. These levels require strategy. Some enemies will attack, so be careful there.
Once again, I'm sorry you've had these problems, but good luck!