Hi! I appreciate your perspective on the auth list. It's interesting; in your case, you're sacrificing performance for visibility and utility. It's a good use for administration.
I was trying to replicate the wallpaper issue yesterday and nothing came up. Maybe it's related to a third-party plugin that randomly generates that. It's an option, mainly because I can't replicate it. I'm also waiting for players to let me know if they see it.
Regarding the last question, whether you build with Ubertool or not, the player can only improve it with BetterTC if you're on the same team. (Well, it really depends on how you have it configured.) But as an admin, your behavior toward the plugin is like another player; it doesn't check "if you're an admin, also allow changing the block."