IMO deduplication wouldn't really be semantically contradictory, because the goal of allowing multiple simultaneous marks is to allow for multiple *different* marks at the same moment. And you could always have a context property to switch deduplication on/off.
The scheme evaluation challenges seem like the bigger issue.
Typically, the way I structure a score is to have a \global variable containing skips + all the information shared across all players, such as key, time, and marks. It goes a bit counter to that structure to either separate out \textMarks into their own variable or to use tagging to control where they display in the full score. Certainly doable, but IMO it would be worth at least clarifying in the documentation.
Thanks for the detailed response!