Thanks @snarfed, brilliant find as I wouldn't have thought of it. I took out the first one which I'd had hiding in a widget and moved into my header for clarity a few months back.

I vaguely recall putting the others in many months ago, but honestly I can't remember for the life of me where they came from (probably IWC) or which portion of my WordPress is actually generating them now. I also don't seem to recall where I was hiding my code that injects my rel="icon" photos (primarily for shortcut buttons on iOS and Android), though I'm guessing they're in the same place. Any suggestions on where a clever guy who's too sleep deprived to find them right now may have hidden them? None of the IndieWeb plugin sub-plugins would be generating them quietly could they?!

The quirky part is that all three lines have been in there a while and (to my knowledge) haven't been causing problems until about a week ago.

The 6 or so outstanding webmentions went right through without any issue, so I'll mark this as closed for now unless problems continue.