I had a similar problem when I was replacing the distributor on my Nissan Quest minivan.
I had one bolt which wouldn't come out, but which I could turn about 1/4 to 1/2 of a turn.
I just kept spraying it and gently, repetitively moved it back and forth for two days, obsessively.
I gradually made some progress, but finally, my impatience and fruatration got the best of me. I took the longest ratchet I had and used brute force until the bolt broke free. It didn't break.
I don't want to necessarily recommend doing what I ultimately did. I don't know if I was lucky or not.
I do recommend continuous, long-term soaking in penetrating oil and working the screw. Eventually, the oil has to seep down the threads and loosen up.
In a sadistic way, I'm kind a glad things like this happen to other people. I'm usually the guy where everything that can go wrong, does go wrong, and a 30 minute job turns into a 30 hour job.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".