This whole blackface stuntman thing always sounded suspicious to me, given no such action is called for in the script, though I suppose it's possible.
The test footage of Klingon cruisers blowing up for TMP is clearly test footage because the ships don't appear to match the hero model Magicam was building.
As to the 1.33:1 aspect ratio vs widescreen, I'm sure they shot it in the latter because it would give them more latitude in how to use the footage in post production.
Pretty sure the Abel pyro tests were all shot in 16mm, so there was no way they were anything but tests (it was very pricey to get highspeed 35mm cameras that could work for FX, so you'd test and work out the right speed to shoot pyro at before booking the super-expensive camera for a day.) Alternately you could test in regular 35mm and then shoot in VistaVision, I guess..
I thought the foam klingon pyro models were cast from the magicam model, but this was the magicam model before it underwent one or two huge overhauls. Remember, Probert mentioned pointing out the neck was misaligned on the Magicam model during his first visist there (right before he became persona non grata there.) So Magicam eventually corrected that, and then Apogee did a ton of extra detailing and changing out of lights after Abel was ousted.
The blackface stuff was reported in VARIETY and/or THE HOLLYWOOD REPORTER; my best friend subscribed to both and I remember him showing me the story after telling me about it. I keep thinking that was discussed a bit here several years back ...
I flashed back on the fact this was a huge issue on LIVE & LET DIE, with white stuntmen blacking up on several occasions, that led to Yaphet Kotto being perceived as a real ass on-set because he kept flashing black power salutes in support of the discriminated against black stuntmen. I think this is all documented in Roger Moore's diary of the making of the film, but I haven't read that in decades ... all I really remember is how he got to talk with Jim Garrison about the JFK assassination and that was the first time I ever heard of Garrison. Moore seemed pretty convinced even back then.