If you can get a little extra clearance, possibly by removing just the radiator, I've had good luck getting out a rounded fan bolt with a Torx socket, and that was with the allen pretty well dug out already by the slipping allen tool. Just a few careful raps with a hammer will seat in the Torx bit, and it has always worked for me. You should also have the tool to lock the fan pulley in place, I assume you are using the lock tool.

I believe the Torx size I use is a T45

Gilly