Re: Dating *e > *i in Germanic

> Incidentally, what is considered the main evidence for early Finnic avoidance of *-o- in
> non-initial syllables?

Aside from the direct evidence in the form of loanwords of the _porsas_ type? This remains synchronically evident in how basic word roots in Finnic generally only allow the stem vowels -a-/-ä- and -e-/-i. Other 2nd syllable vowels only appear in loanwords; derivatives; or in inflected forms (though by now, perhaps well-obscured without etymological study, since o-stem loans *were* being adopted back in Proto-Finnic circa year zero, already).
Some typical derivation/inflection patterns:
_pala-_ "to burn" > _palo_ "fire, conflagration"; _paloi_ "(it) burned" (*aw > *o, *aj > *oi)
_pese-_ "to wash" > _pesu_ "(act of) washing"; _pesin_ "washing-brush; I washed" _pesty_ "washed" (*ew > *u/y, *ej > *i)

I don't think the earlier "avoidance" of primary o-stems etc. in loans can have been purely phonological for all of its lifetime, and probably mostly represents assimilation of loanwords into native inflection types. Also, I don't think the origin of this can be quite placed all the way in Proto-Uralic. Most of the secure early IE loans (like "pig") are either relatively limited in distribution or show irregularities that suggest having been acquired in parallel into various Uralic branches. The most archaic lexical comparisions (like "me", "water") are OTOH hard to show to be loans at all.

Some other direct examples of "vowel flattening" in loanwords:
_ankerias_ "eel" < Baltic *angurjas
_kirves_ "ax" < Baltic *kirvis
_sisar_ "sister" < Baltic *s(w)esoor
_mesi_ (_mete-_) "honey" < PIE *medhu-

> Most of the IE branches that have uncontroversially contributed words to Uralic are also
> branches in which short *o generally shifted to *a -- Slavic is the only exception that
> comes to mind right now.

And it's a later development even there. Early Slavic loans in Finnic such as _akkuna_ "window" (Ru. _oknó_ < *akUná), _taltta_ "chisel" (Ru. _dolotó_ < *daltá) still show retention of all sorts of stuff like PBSl short *a, yers, liquid diphthongs, nasal and long vowels, etc.

_j.