Yeah, I def. need a level before 10 to explain what that symbol means. If you want it spoiled, it’s a door that opens when all other doors in a room are open.
Yeah, this was true for me when I was first designing the puzzles too. There are also a lot of layers of thought you have to operate on. Like, what are you trying to accomplish at a high level (e.g. what do the teleporters theoretically let you do that you can’t do without them?), and do the low-level specifics allow you to do that or not?
Specifically Sokoban where the available space around you can change (and is obfuscated), so I wouldn’t beat yourself up too much for that.