Seized rear derailleur jockey wheel bolt

A mixture of ATF and acetone is supposed to be the best penetrating fluid there is. I made two bottles recently in a fit of enthusiasm so, if by any chance you're anywhere in the Manchester/Liverpool area, you're welcome to a bottle!

Penatrine (sounds as read not sure regarding spelling) is a really good fluid. Spray as best you can into thread and leave to penetrate overnight. Penatrine pretty decent if really stuck. Can lightly tap head to loosen up a bit. No Thor strength required, just a tap tap.
Easy outs as suggested. Locking pliers if rounded. Both. Either or.
Main guts is to get lube into thread. Soak as mentioned or spray and let soak in.
When bolts are tight and won't budge, often it pays to crack there threads by initially tightening them just a smidge. Loosens threads by tightening, get oil into them and then loosen the bolt. Add oil first!

Soak in plus gas overnight and then with top pulley bolt undone, try rotating the whole rear cage plate against the seized bolt.

Remove the derailleur and soak the inner (spoke) side in penetration oil as suggested. Leave it an hour or two, maybe overnight. Doesn't always work but it's just a few threads on that side (at least it is on Shimano mechs) so shouldn't need much loosening.

If that fails, escalate to the alternative above. "Easy out" sounds good.

on the bench..... use an "Easy Out".... screw extractor.....we use them all the time(mainly of FSA crank bolts as they are sh*t and soft alloy) ... use the pilot drill and then lightly tap in the screw then turn it anti clockwise... and the bolt will unscrew.... pop a bit of grease on the thread when you reassemble

You could drill the head out of the existing bolt. Use a bit about the same diameter as the shank of the bolt. Once the head pops off you should be able to rotate one of the plates aside and get the remains of the bolt out with pliers. Have you soaked it in penetrating fluid yet, that might help?