1. ...(Where did the term "speed metal” originate, exactly?)
Fanzines - in other words, from within the subculture itself, rather than being imposed by the corporate glossies seeking to max out ad revenues.
2. As a description of a particular scene or style of music, “thrash” was still barely defined by mid-Eighties (and predates Anus favorites like C.O.C and D.R.I.), which is probably why even a traceable, press-created term like “crossover” became roundly accepted at the time.
D.R.I. and Corrosion of Conformity were both well established by 1983/84, when 'thrash' (speed) metal took off, so I'm not exactly sure what point you're trying to raise here.
The picture overall is a lot more complex than is typically appreciated, in any event. There were three or four separate genres emerging from pretty much the same primordial stew of punk, hardcore, and heavy metal. Apocalyptic Raids
and Ride the Lightning
are pretty much exactly contemporaneous. Death metal emerged pretty essentially fully formed by the summer of 1985. The early grind bands hadn't yet managed to get record deals, but grind was certainly already extant and viable by the same time. In the three years between spanning the summer of 1982 to the summer of 1985, the extreme edge went from Iron Maiden and Discharge to Sepultura, Bathory and Genocide/Repulsion. That kind of evolutionary pace is impossible for the language to keep up with, except in retrospect.