I-ÚHINNEN ENETH IARWATH The Unknown Name of Iarwath

Sorry for so many questions guys but that's how I learn by doing. So for this one looking in the gloss we have the adjective sinnen (neologism for known) which is a past participle of the √IS root. “know” or verb ✶ista- v. “to know”. and the prefix ú- pref. “no, not, negative; impossible” and since pref. negates the sense of the following word. And is applied to all classes of words but especially to verbs and adjectives. Can it be applied to sinnen using a soft mutation to become úhinnen (not known) essentially "unknown"? And as pointed about before there should be no need for na between eneth and Iarwath being in front show the implied relationship.

Úhinnen is fine. (The prefix ú- was later rejected by Tolkien, but it still is the best attested prefix. You could alternatively use al-, but that is definitely less attested.) Also reverse the order of eneth and úhinnen; adjectives always follow the nouns they modify (except for numbers as adjectives, which precede nouns).