I couldn't verify if this works with mbstring, just assumed it did, and only verified it in Fallback.php by running it.
Now you are confirming the same problem with mbstring.

Core wiki functionality.

That is if people have the habit to look at the tooltips. Not many websites have meaningful tooltips (ebay doesn't, facebook doesn't), so people don't even read them. (BTW, tooltip doesn't even show when mouse is over checkbox, only when it is over text.)

I think it is also beneficial if it is near existing "Disable access keys" checkbox, which is in "Gadgets". Because related options better be together. Maybe put it there in "Gadgets"?

Once mediawiki rejects new account due to the name similarity, it should offer the user to consolidate login IMO. Ask for the password for that other login, and ask to offer to consolidate, if consolidation is a desirable in general.

No way to attach anything here for some reason, here is the hex dump.
You can see that Content-Length is 20, and content is trash instead of text/html.