You don't say where you are - cos you might need someone to come round and help - seeing it is sometimes the only way to _know_ what will do the job.
Heat will often help to move something that is siezed - but you do have to have a grip of the bolt
Have you tried an allen bit in an impact driver ?
I've done things like saw a slot then use an impact driver, and sometimes I've found an imperial allen bit that can be persuaded to fit.