On pg. 148 I say that the fate of final -a (from any potential PIE origin, i.e. *-a, *-h2e or *-h2) is unclear. I cite the case of ita, which, if equivalent to Ved. íti < *ith2, would show preservation of final -a. However, I also note the possibility that it could come from *itā with Iambic Shortening.
This hesitancy is inconsistent with the view I endorse firmly elsewhere that the athematic neuter nom./acc. pl. -a is from < *-h2 (e.g. p. 212). If the latter is true, then the outcome of final -a is definitively known. But this last fact is not absolutely certain. In Sabellic it is clear that the thematic ending -ā < *-eh2 was generalized to athematic forms as Umb. tudero 'borders' VI a 15 (where tudero might be standing for /tuderof/, but the extension of the animate acc. pl. ending -f to the neuter presupposes a form tudero). If this happened in Proto-Italic, independently, or by diffusion in the prehistory of Latin, and if there was a phonological shortening of final -ā, then the athematic neuter nom./acc. pl. would not be informative about the fate of final short -a. But since trīgintā etc. seem to show that there was no shortening of final -ā, this scenario seems problematic.
In a recent article George Dunkel (2008) argues that final *-a became -e on the basis of the suffix -ne, as in superne, supposedly from *-na, which seems to match Hitt. ištar-na, Neo-Phrygian ενσταρνα. In addition the Umbrian form perne 'in front' is related to the adjective pernaio- and the most straightforward way to do this is to start from a pre-Umbrian *pernai. This is an idea I have toyed with myself, but I don't feel confident that we can exclude a particle of the shape *-ne.
Dunkel also re-proposes an alternative etymology for nōn, i.e. by apocope from *nō-na, the *nō- of which can be compared with Hitt. natta 'not' < *no-th2(-oh1). This is attractive since the traditional etymology from *ne-oinom requires a lot of special pleading.
Dunkel, Georg. 2008. Chips from an Aptotologist's Worshop II. In Brigitte Huber, Marianne Volkart, and Paul Widmer (eds.), Chomolangma, Demawend und Kasbek. Festschrift für Roland Bielmeier zum 65. Geburtstag. Band II Demawend und Kasbek. Halle (Saale): International Institute for Tibetan and Buddhist Studies, 403–12.