"Who would say "m'kay"?" Not me! And nobody I've ever met, either. In fact, I've just had to google this to find out whether it exists at all. "I'm wondering if this is for everyone" No. Definitely not. This may be common in some circles, but it is certainly not 'for everyone.' It appears to be a feature of some kind of American subculture, popularised by the cult cartoon South Park. Personally, I have never heard anyone - either young or old - say this any context.

Yes, it is an informal way to say ok. I would say it is pretty commonly used. Person A: Let’s meet up at 7, my place. Person B: M’kay, sounds good. See you there!