Hi watcher615u3
First off, sorry to hear about that frostbite thing. That sounds painful.
Yes, the "blue hitbox" on some players is Wurst's friend system. Any player you mark as a friend will not be attacked by combat hacks like TriggerBot. You seem to have figured that out already. But as for Wurst "randomly" adding and removing friends, I think I have an idea what might be causing that.
The friend system has a special default keybind called "Click Friends" (or "Middle Click Friends" in older versions). This makes it so that whenever you press in your mouse wheel while looking at a player, that player's friend status gets toggled on or off. This is normally quite useful, but I can see how a hand injury would cause you to accidentally trigger this keybind all the time, leading to "random" friend changes.
Here is how to turn that off: Open the pause menu, go to "Wurst Options" and then click on the "Click Friends" button until it says "OFF".


Hope this helps!