Zompist Bboard Again

How well reconstructed is Pre Proto Germanic and how far does the reconstruction get relative to Proto Germanic of ca 200 BC-0 AD?

Also when exactly did it split and became Pre-Proto Germanic as opposed to Northwest Indo European?

Effectively, you're asking "How far back can we reconstruct Germanic with confidence?" I'm pretty sure the answer is "Not much further than about 0AD", but I'll leave it to others to be more precise.

Germanic underwent some idiosyncratic developments setting it off from other IE branches, the most salient and most famous of course being the Germanic Sound Shift ("Grimm's Law"). It seems as if what was eventually to become Germanic was isolated from closely related dialects (Balto-Slavic) for many centuries. In my "Northern vs. Southern IE" model (according to which all modern IE languages descend from "Northern IE", essentially to be associate with the Corded Ware culture, while Anatolian and the lost language of the Bell Beaker culture descend from "Southern IE"), the language ancestral to Germanic was brought to southern Scandinavia by the Corded Ware people early in the 3rd millennium BC, and cut off the Northern IE dialect continuum when the Bell Beaker people, who spoke Southern IE, pushed back their Corded Ware cousins from the Rhine back to the Oder a few centuries later. This gave Germanic about 1,000 years to develop in isolation from the rest of Northern IE, until it expanded southward after 1500 BC (Nordic Bronze Age) and eventually met Proto-Celtic in central Germany in the course of the Urnfield expansion around 1300 BC when Italic (south of the Alps) and Celtic (north of the Alps), originating in the Pannonian Basin, clobbered the Southern IE languages descended from the language of the Bell Beaker culture. As for the dating of the Germanic Sound Shift, it probably was only completed in the Iron Age as the oldest Celtic loanwords in Germanic were affected by it, but it may have been prepared by aspirating the *T set and devoicing the *D set (like the Armenian Sound Shift, which is of course an entirely independent development, there never was a "Germano-Armenian" node) during the centuries of isolation of Germanic. Verner's Law may have happened between the two phases of the Germanic Sound Shift, i.e. it turned aspirated stops into breathy-voiced ones. All that, though, is just speculation; for instance, nobody knows if any "Southern IE" languages besides Anatolian ever existed!