I got some of that — supposedly excellent — Gorilla 2-part epoxy.
Will someone tell me how to use this when reattaching a car indoor mirror
without excruciating pain having to hold the mirror up for 5 minutes straight?
I can’t do it! And by the way, I came close to doing it and it still didn’t hold.
Yes, I followed the instructions perfectly.
Thanks to anyone who can help.
Pretty close to taking a sledgehammer to
the windshield at this point.

I haven't tried using the Gorilla glue 2-part stuff, but I've used Permatex's Five Minute 2-part epoxy, and it has worked.

Anyway, I digress on that matter. I'd recommend maybe using a piece of tape to hold the button in place while it cures. I'm sure you could attach it firmly enough to keep adequate pressure on it.

Yeah, you are supposed to take the button out of the mirror, not hold the entire mirror up.
The metal piece can be taped to the windshield to hold it until you get back to it 10 minutes later or so, you might get some drip down the window, but you can clean that up later.

I use this Loctite 37438 Rearview Mirror Adhesive follow the directions on the package. Before you clean the old spot where the mirror was go on the outside of the windshield and Mark the location with a marking pen. The clean the old spot off on the inside. Use the primer the comes with the kit to prep the glass open the adhesive tube and put just enough on the metal piece to cover the bottom. Then stick it on the spot and hold it for about a minute. The stuff is contact glue (cyanoacrylates) it will set real quick. I like to let it cure for a couple of hrs before attaching the mirror.

Locktite kit works great. Lay some 800 grit on a flat surface and rub the glass side of the button on it to get rid of any glue residue and scuff it up a bit. Don't overdo it, you're not trying to plow gouges in it.