PGmc adverbs update
I have not been able to find a definitive source for the etymology of the ablative *-nV suffix, but apparently it could also appear in Sanskrit vinā, "without", and Latin supernē, "from above". The suffix could also apparently show up in Germanic *fanē, *hinē, *þanē, according to Kroonen. The former could be derived from PIE *h₂po-, a variant of *h₂epó, which would make it cognate to Latin po-.
If you find it acceptable, we could create the entries with the *-ē ending, with an explanation that the vowel is indeterminate and that that form is either the least problematic or least contested form. If not, then we could take up your earlier proposal to create the forms with the vowel left blank.
I found a paper recently that went a bit further into various Germanic words with an n-suffix, including these adverbs and prepositions. It's called "The Proto-Germanic N-stems - A Study", by Guus Kroonen.
There is another paper available on academia.edu titled "Consonant Gradation in the Proto-Germanic Iteratives" by Guus Kroonen as well, which I guess would be a shorter follow-up.
One thing I was wondering about. Should not Proto-Germanic *ana have lost the final vowel?
Is it possible that there were two originally distinct forms, *an(from earlier *ana) and, say, *anǭ or *anē?
Final -a is not preserved in any Germanic language that has a body of any decent size. It's only found in Proto-Norse runic inscriptions, and those attest only a few words.
As far as I know, the preposition was just *an. The final vowels were proper to the adverbial forms, which is detailed in the paper.
I'm sorry, I have confused myself here. So the preposition *ana, ("on, onto"), should only be Proto-Germanic *an? And I meant, did the form giving Gothic, Old High German, Old Dutch and Old Saxon *ana, ("on, onto") come from a variant of *an, possibly adverbial?