For (1) (irony), I really like your translation “so much for….” For number (2), your ideas are good, although I believe you may be over-thinking it — perhaps it would be more natural, and more in keeping with the everyday nature of “menos mal,” to simply say “it’s a good thing that….” This also preserves the grammatical structure of the Spanish, by using a subordinate clause instead of an adverb like “thankfully,” or an interjection like “lucky you.”
I believe there’s also a third possibility, between irony and seriousness, which I would translate as “It’s just as well….” You might also use this for a somewhat less ironic interpretation of “Menos mal que iba a llover” — “It’s just as well it didn’t rain.”