When arguing for *anqua, I would mention hequa, an actual adverb/preposition that's formed similarly from a K-final root.

I tend to distrust isolated roots without any derived forms, especially since (A)NAK “bite” is already very well established, and the most likely derivative of primitive *anak- would be anca, but that’s already attested as “jaws”.

I think a derivative of (N)DAN “back”, analogous to S dan “back to, against” would be a better choice. We can’t use Q. nan as “against”, since its already attested as “but, on the contrary, on the other hand” (and in only place “again”, but I’d wouldn’t use it this way).

Maybe ᴺQ. nanna “opposite, against”, both adjective and adverb? From (n)dan+nā or (n)dan+nnā?

+Paul Strack I do not mind the occasional isolated root, homonym, irregularity, they spice the language.
nan- for me is too heavily associated with backwards movement both literally and figuratively (as in nankar-), but I guess the meaning could get extended over time.

+Tamas Ferencz Actually, upon further reflection I’m starting to warm up to anqua, probably from anak-kwā. I do like the idea that some forms are distinct in Sindarin and Quenya, and the nan... space is already very crowded in Quenya.

Glad that you can be won over:)

+Tamas Ferencz Anqua sounds to me much better Quenya than anat, and I'm adopting it straight away.