The Americans who know when to use whom and who and Americans who think they know when to use whom and who are those who use whom, while Americans whom the distiction between who and whom thouroughly confuses and Americans to whom whom is entirely unknown are those by whom who is solely used.
Oh it's a bit more complicated than that. There are also those who don't use whom because they know it's a relic of a case system that has been gradually fading for a thousand years. Not to mention those whom use it incorrectly on purpose to annoy the pedants.
Kudos for writing the entire sentence without making a mistake (yes, I checked). Although perhaps the use of 'whilst' would complete the intended stylistic flare?
> as an American a sentence like "Whom did you invite to the party?" sounds a bit stilted and formal to me
It would be fair to call this ungrammatical in American English.
But whom does survive in fronted prepositional phrases ("the person for whom this item was obtained..."). It's dead in prepositional phrases that haven't been fronted just like it's dead everywhere else.
Something vaguely similar happened in Spanish, where there is a special pronoun case that can only be used with the preposition con ("with"). There, the special case descends from, interestingly enough, the same preposition, Latin cum, instead of from the Latin case system. But the phenomenon ends up being the same.
Yet, among those with whom I associate, "Who didja..." is more common than the semantically equivalent elision "Who'd you", probably because it is ambiguous whether the latter is in the past or present tense.
(For context, I live in south-west England, have an RP accent, and 'whom' was genuinely the word that felt most natural to me when writing this post.)
People speak of the future conditional so rarely, it doesn't affect linguistic evolution.
"Whodja" being shorter and less stuttery than "whodidja" is a much more powerful driver. Like "Wensday", unless you actually need to disambiguate past from future.