Yeah, it's a pretty common mistake to feel that anyone owns a wiki.
It's far easier to upload your image via the fandom app as it re-sizes your image to appropriate dimension before the upload. So, I suggest using the app for it.
It's for announcement purpose. Fandom is introducing new rules and blocking policy with effect from April 1 so they are advertising and spreading this to all the wikis via this notice. It's not a bug or something no matter how much annoying it is. You just need to hit cancel button to get rid of it. Hope it helps!
Hey creating another account to evade a ban is considered sock puppeting which is prohibited on fandom. Most probably they identified your sock and blocked it as well.