Thanks for the advice all! Tried some of this last night. Had added more penetrating oil over the last couple days (didn't post until after I'd tried the freeze-out yesterday)

Vice grips just couldn't get enough purchase, either straight on or turned to the side sadly. Again wasn't using my brain, this bolt was way too tight in there to begin with. I guess it's my way back German heritage of getting something guten-tight.


Filed a slot with my thinnest needle file from the set. Tried it shallow and no-go, every screwdriver or bit I had slipped. Filed it down to almost the dropout. Sides just weren't up to the torque (tried turning either way) and they started bowing/twisting out.
EDIT: Seems obvious now, but I wonder if I could've grabbed a thinner M5 nut, carefully threaded it on the remainder, THEN used a small screwdriver bit. Nut would've maybe held the "ears" together. Something to try if there's a next time...

Filed off the slot just about flush and started drilling. Too many dulled and snapped bits from the good set over the 15 years or so I've had them, so I need to run to the store and I'll grab a set of bits for metal. I have a set of Easy-Out type removers, but was going to take a peek at what they have at the store too.

Planning to drill out incrementally and try the extractors I have to the extent possible. May try warming up the whole dropout with the heat gun (expanding under heat), then hitting the hollowed out bolt inside with freeze-off (contraction), then an extractor. Seems like maybe the best combo of everything at my disposal at this point. Due to how tight it was to start, I'm not holding my breath it wont just have to be drilled out completely. But I've got to try right? Oh the indignity of having one threaded mount and one smooth bore!

The opposite side of the snapped bolt is nearly flush with the dropout, not recessed, but not even a full thread showing.