Is this frame trashed?

[Big Hammer]

Well if we’re being silly. Set height. Drill through post and frame. Insert bolt. Job done.

There's nothing silly about it at all, I was about to post suggesting exactly the same remedy. If the two realistic options available to you are:- 1) chuck it in the skip, or 2) engineer a cheap but unpretty bodge and keep on riding it, then in my book the second option wins hands down.
You've got absolutely nothing to lose in a situation like this.

Just do the drilling with the bike angled or inverted so the BB is higher than the crossbar, so the swarf doesn't get into the BB bearings.