Is this compound sentence grammatically correct?

Yes, the sentence is grammatically correct; here is a syntax diagram (parse tree) for the sentence:

It “untangles” intelligibly, if that’s what you’re wondering. Obviously you wouldn’t ordinarily use a run-on sentence like this one in formal writing, but it’s sometimes done for artistic purposes, as this clearly is.

No that sentence is a run on. There are also several missing commas in what would be the sub-sentences, and past/present tense issues.

I would revise to:

They figured, since I’m a real person and I’m in the movie, that I should play myself. I didn’t even know yet that I was me in the movie, although I did know I was me but no as myself in the role. They had better let me know that I [was] am in the movie playing myself and let me see if I would be okay with being me in the movie now that I [knew] know there [was] is actually a movie with me in it.