Does making it legal tender mean stores have to accept it there (that's not a thing in Europe so idk - you can get turned away with 50s in the UK if they're not comfortable accepting them).
I doubt that. It's the same thing in the US with larger denomination bills; merchants can choose not to accept them if they don't want to. Making something legal tender just means that businesses
can accept it for payment if they want to, not that they
have to--but again, that's coming from my knowledge of how the US does things. I don't have any idea what it would mean in El Salvador, but I don't think it means anyone's being forced to accept bitcoin.
I'm not surprised that bitcoin isn't a very popular alternative to cash, even in a country like El Salvador. As OP pointed out, its volatility means that it's generally not a great hedge against a fiat currency that's failing--at least not in the short term. That's why I'd never want to accept bitcoin as an alternative to cash for my paycheck. The week-to-week swings in value make it impracticable.