Yeah, 200GB is not normal. Sounds more like you at some point clicked “select all” and then “install” in Synaptic. (This kills the Debian)
Yes, you can install different DEs without conflict.
But manually and individually removing all packages you think belong to one DE will lead to breakage. XWayland is like a compatibility layer that lets programs designed for X work in Wayland.
Yes, if you install and start Gnome, you’re using Wayland. Programs that can’t will use XWayland. You don’t have to worry about it.
Then google how to reset the BIOS password on your hardware. Sometimes it’s a jumper you can reset, sometimes you have to take out the CMOS battery, sometimes you have to call the manufacturer and provide proof of purchase.