If by figurative you mean similar to when we say يعطيك العافية we mean hello, and when we say الله يخليك we mean either please or thank you, then I’m really not aware of any dialect that uses it to express a certain sentiment.
If you mean any hidden meanings in the phrase itself, then there are none, it’s quite literal.
Correct. The past is very commonly used for wishes, prayers, invocations etc. Think of the famous صلى الله عليه وسلم 'may God praise and preserve him' or رضي الله عنهم 'may God be satisfied with them' or حياك الله 'may God give you long life' or عاش الملك 'long live the king'.
The muDaari3 on its own, I think, could only mean 'God blesses you', not 'may God bless you'.
Of course I have. I'm not sure this use was entirely correct in classical Arabic, though - it strikes me as a dialect influence (where prayers like يعطيك العافية or الله يرحمو are very common). I'm happy to be corrected on this, but even if I am, the fact remains that it's very, very common in fuS7a to use the ماضي to express wishes and prayers.
Well, as I said, I'm happy to be corrected on it being a possibility in classical Arabic. But that's not really relevant to your question, which has been answered: the ماضي form is very commonly used in invocations and wishes, and that's what's going on here. I could equally ask you 'have you never heard someone saying صلى الله عليه وسلم'.