Had difficulty with some seat hardware once, really stuck after 50 years. Definitely different animals but the solution was an impact. It was a screw head with regular, not philips head. I used all the usual tricks and finally after serious frustration had an idea to use a center punch on the outside most edge of the slot and strike it with a hammer in the direction I wanted to go. Finally it gave.

Maybe drill a small pit in the top of the bolt you want to budge and use the shock from a punch and hammer to "crack" it.

Good luck

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63 220S W111
76 300D W115
2013 VW JSW TDI M6

previously-
73 280 SEL 4.5
86 300E 5 speed
2010 VW Jetta TDI M6