So we have been iterating fast, adding objects, removing objects, adding entire new versions in parallel of imports and changing materials and re-importing a lot.
And what a bloody nightmare packaging has turned out to be!
Unreal constantly complains about missing file, yet those files are deleted from the level, deleted from the contents folder, obliterated from Perforce and still we can’t package!
And the fact that you have to dig into two different .txt files makes this extremely difficult, because the Unreal Editor doesn’t complain when I play from within it, so why does the packaging do it?
(Also, it’s super annoying that if you delete a Datasmith imported object from a level, you get popups warning you that you will delete references. So what? What references? I place objects and delete them all the time in my levels, why doesn’t Unreal handle the references for me? Also, I just deleted the objects from the level, saved, and now I want to delete it from the project. Why does the popup window still claim that the level is referencing the object? It’s not true! I just deleted it! It’s gone from the level! Nothing should reference it! Someone said that Unreal is made for games that are hundreds of gigabytes in size with team sizes of hundreds of people, and it sure shows because compared to Unity, it’s a pain to work with when you’re just a small team.)
PS. I know about “fix up references” now and I try to click it all the time, even though it doesn’t really visually do anything (but I’ve double checked in File Explorer and the files are gone).
EDIT: Wow, this seems to be a common problem, possibly made worse with Datasmith…
https://answers.unrealengine.com/que…ent-asset.html
https://answers.unrealengine.com/que…find-file.html
https://answers.unrealengine.com/que…r-asset-1.html
https://answers.unrealengine.com/que…ml?sort=oldest
https://www.reddit.com/r/unrealengin…ile_for_asset/
Nothing has helped so far and I don’t want to recreate all those file just to delete them again as one suggests… someone said to check “full rebuild”, and when I do, I now instead get “source art in an invalid format” for a texture imported via Datasmith.
EDIT 2: Testing on a new file including the Vray material with the offending texture, I do get a “some texture maps used in the scene are not supported by the Unreal Datasmith format in the moment. They will not be imported inside Unreal Engine.” yet they appear to be, and causing problems? Or is that what Datasmith always say about Vray bump maps, because it’s actually the basecolor map that the packager complains about…
EDIT 3: Created a test project with just that one Datasmith object in it. It now imports with black textures marked “invalid texture”. Thank you Datasmith. 
EDIT 4: Great, so now I get a repeatable crash on re-import with Datasmith after replacing the “invalid” textures in the Vray material. grumble grumble