I don't actually agree with your parsing of the word, but that's not super important. Sentinels don't have to be passive observers, and if they aren't passive then what are they doing? Probably something that sounds like investigating with a soupcon of doing so to protect someone or something. You get
guardian of or
defender against as easily as watcher out of Sentinel, and both the former describe the class really well. Can you call a Witchunter a sentinel against the rot of chaos? I think it fits, and they wouldn't be much bloody use if
all they did was watch.

Anyway, that was my thinking.