I think you're correct to say that including "a" in all the examples slightly changes the meaning of the phrase. Both a'ed and a-less seem grammatically fine. I would also say that "hate with a passion", specifically, is an idiomatic expression, so somewhat immune to routine rules. You done good. It ain't over till it's over.

I would say that with the use of "not only", the "also" is simply redundant, and so unnecessary. The "but" might not even be needed, though just being concise for its own sake isn't always the best way to go.

"Not only did George buy the house, but he remodeled it."

I think either would be okay, grammatically. As others have said, in this case "was starting" might be more specific and therefore informative, if in fact the film was just beginning. Saying "had started" is more ambiguous about how long it had been running. If it had been going for, say, half an hour, you probably wouldn't say it was starting, but might still say it had started.

Just curious, but what is "resubstantial" intended to mean, in context?