Should tMR allow .blend files?

[Sir Teatei Moonlight]

I'm not sure where you got this idea, but the DAE format explicitly supports normals. Search for semantic="NORMAL" in the specification doc.

I admit I hadn't dug too deep into this. All I know is that when I round-trip .blend -> .dae -> .blend, I lose the custom normals data, and I see no related options in the importer/exporter. (Or at least that's how it was last time I checked. Something could have improved since then.)

for cases where importing multiple UVs is not possible.

(emphasis mine)
Okay I had misremembered this one, I got it backwards. (Though I dunno it seems like a pretty big issue if your program of choice can't import that properly these days.)

One situation that I very much want to avoid is people relying on Blender for handling everything difficult. The last thing I want is for someone to download a ZIP and the readme says "Animations are in the Blender file" or "Other UVs are in the Blender file" or "Vertex colors are in the Blender file" or "Bones are in the Blender file" or even "Textures are in the Blender file".

I completely agree. If the secondary format is removed, the rest of the submission should still be valid by itself.