Possessive suffixes in everyday language?

[PäähäniSattuu]

Okay, from what I understand, possessive suffixes are mandatory in formal literary style but no one speaks formal literary style and you look like an idiot if you speak it rather than writing it in most situations or something. Their use in spoken language is a bit complicated. But my feeling seems to imply that they are mostly used in situations where everyone has exactly one of (or one set of) such as äitini, taloni, pääni, äidinkieleni, sydämeni, silmäni (implying plural) etc. Is this a correct reasoning, or not at all, or does this differ per dialect?

[Pursuivant]

Differ per dialect, I'd say. Also using some archaic forms so instead of 'poikani' say 'poikain'... Most common though would be using forms like 'mun auto' and 'meidän talo'... Just need to listen to kids 'meidän isä on isompi kun teidän isä'...

[Valinnan vapaus]

I think I'd forget about the theory of having one or one set making the difference.

Whenever I'm in a certain mood I can use a bit more formal/literary/poetic language like "Tulipa tuosta mieleeni". Also some expressions are idiomatically taken from literature and therefore the literary forms are kept. Maybe something like "sydämeni pakahtuu onnesta" where you can tell it's like that because "pakahtuu" is not such a common word.

btw it means "my heart is bursting with joy"

[noxie]

From my experience, I don't hear them that often, in spoken language. I think they are much more used in written language. I would rather say 'mun äiti then 'äitini. Either way, I would be understood.

[Rip]

and you look like an idiot if you speak it rather than writing it in most situations or something

I would not worry about that too much. I'd say fake colloquial style Is something I'd notice (in negative sense) much more clearly ("colloquial style that does not flow naturally).
Having said that the for, I think forms like Minun kirjani (Where in non-casual writing "minun" is optinal and "-ni" is mandatory), is rare (at least in my experience, may indeed vary based on location and age). You drop out one or the other (and of course typically use 'mun' or some other variant of 'minun')

But my feeling seems to imply that they are mostly used in situations where everyone has exactly one

I think there might be something here related to here how you would express it formally. It depends on situation, but I think you'd be relatively more likely to say (or write) 'minun kirjani' but 'äitini' (less emphasis needed maybe, it is sort of like more clear to begin with, which of the billion + mothers in the world you're talking about?) . In spoken language you usually make things shorter, so the first example is often transformed to "mun kirja", but the second one you'd actually had to make longer for the identical form (assuming some kind of possessive structure would be required), so it might be more likely retained with the suffix.

Obviously this is no clear rule, and if you stop to think about these when you should be talking, then more fluent (flowing) speech, even if formal, would have been a better option.