The proposal is very well written, as always Walküre, which makes it more difficult for me to say it, but my vote is a nay. I'll explain why:
We are talking about beings not know even to some of the most powerful maiar, so that, as you say, could even precede their arrival to Arda. They have been nibbling the Ambar probably before other beings like, for example, Ungoliant, stablished on the planet. They should be really powerful, excedingly so, as tolkien uses to do with old things, so I think that they won't fit in the gameplay, much less as a third tier spell.
These beings have not been seen by anyone, ever, except Gandalf and, probably, the Moria Balrog. Having them appear in bright light in the middle of combat would be even more exceptional than seeing a Vala.
We now nothing about them, it's not even neccessary that they look like worms or "something simple": Ungoliant looks like an spider and Tom Bombadil looks like a man with a hat, and the origin of both could be as misterious as the nameless things one.
The worm in the game hints clearly to a modified subterranean dragon, and, after reading above, we could think that that is not important, but dragons are linked to Melkor, so it would be nice to clearly separate a "nameless thing" from this reptilian group, this could be fixed with a model change.
We don't even know if the Watcher in the Water is a nameless thing, as is related to the depths of Moria and Gandalf also doesn't know what it is, so we could be even duplicating on said nameless things. (this could be in conflict with the first argument as the Watcher is a Tier 3).
I would extend more but I'm in a hurry, sorry for this abrupt end. Is a good concept but is way too lore breaking for me.
Cheers!