Parts of your argument don't make a lot of sense. You've itted musicals worked in the Arrowverse. So let's take a look at that more in depth. The Flash/Supergirl musical crossover didn't have much of a relationship to past or future episodes of either series. Darren Criss was a 'villain' (or at least a morally ambiguous character ) without much of a backstory, and who never showed up on either series again. There's no reason why Smallville couldn't have had some kind of similar plot to get the characters singing. So really the only major sticking point for a musical Smallville episode would have been whether the cast could sing and dance.