Mother Winter can destroy any enchantment
Yeah, but she doesn't get involved.
There are many more actively worshipped gods stronger than Ethniu
I dunno. I think the gods are mostly either far-diminished (like Odin and possibly the Lords of Outer Night, if they ever really were gods rather than faking it) or distinctly not intervening in the mortal world.
And there may not be as many out there as it seems, given the possibility of one being holding multiple mantles and names (Odin and Kringle and Beowulf are all Vadderung) and cross-overs between different mythologies (Grendel was a Bigfoot). Vadderung might also be Mercury and Thoth (historically the ancient Romans considered Wotan/Odin and Thoth to be Mercury worshiped under other names).
Not sure how much worship has to do with the amount of power a being has. I think that's a feature of other fantasy universes, not a Dresdenverse thing.
Knowledge of a being can be relevant to give a being access to the mortal world... but belief isn't required, much less worship - knowledge of it in a fictional context is good enough, from "Backup".
Ethniu seems stronger than Odin in PT and I don't think she was really
ever worshipped even in pre-Christian Ireland; the Fomorians were more monsters than gods. If she were, her worship certainly would have died out long before Odin's. (Ireland became Christian well before Scandinavia.)