well, just to keep the ball rolling:
The borrowing in English of the Spanish word "mucho" is very common, in phrases like this. "Mucho X" means "much x" or "a lot of x." It's a commonly heard slang or informal usage.
So here we get the meaning "much mistrust," which makes sense.
"Much o' mistrust" would mean "much of mistrust," which isn't even idiomatically correct English, and I don't think any native speaker (US, anyway) would naturally use this expression in this way. At least, I don't think I've ever heard it.
[Edit: I'm actually not sure if "much of mistrust" is grammatically acceptable or not. I can just about imagine it being used in a poetic or literary context, or somewhere where a bit more flowery or elevated tone is desired. But not in ordinary conversation or usage.]
If she wanted to use the English "much," she could have just sung "much mistrust," a much more natural way to say it.
Listening to the song, it sounds to me like Ms. Harry uses (an American version of) the Spanish pronunciation of "mucho," and not the AE pronunciation of "much."
The website Genius gives the word as "mucho."
Re "gas": I think it would still be understood today and I wouldn't be too surprised to hear it used this way in ordinary conversation in 2021, but probably not by younger people.