TobyMac

It's not that the MOC will directly be rejected when you submit it. It's more as a warning system to: a) Check if your inventory is actually complete. Especially a first time it can be tricky to make an inventory. b) Rethink if you want it to be a premium MOC. We often get small submissions, where you can easily reverse engineer the build just by the main photo. (in the old system with self-hosted instructions, there was a starting fee, independent of how many copies you would sell. So submitting a MOC that clearly wouldn't sell was not smart) c) An extra reminder for the admins to check if everything went OK with the submission.

I think there is no app for that (yet). Only desktop

In the admin tab of your MOC page, there is an option to put in a link to bricksafe.com. On bricksafe you can upload your photos (free), and RB makes a slideshow of them for you.

Weird: I was looking for an image of 8880-1 as an example, but all set images don't have the steering knob on top

for instance these models:

Ok

Do you have Alternate Parts activated? If so, you might have part 62531 in your collection being used in the calculation.

This also solved the issue with approving Fan Photos for MOCs. The approve button disappears after approving.

Firefox is known to have some issues on the site. Can you replicate the problem in a different browser? To be clear: the part does not show up at all as being missing? Or is the quantity not correct?

That's why I also have a standard test account. I often forget which options are only in pro or admin.

Nitpicking: order of appearance?

I found out the hard way. Had a very frustrating hour trying to figure out why the axle on my 8043-1 wouldn't fit. The parts got mixed up with an older set.

@Nathan An example you can use for testing: Normally, when I approve a Fan Photo, and I open the photo pop-up again from the admin page, the button Approve Photo is gone. Since a few days the button stays in the pop-up, like nothing has happened. When I refresh the page, the photo is gone from the list, like it should be.

Which 13 are those?

I've tested a random 10 printed versions, and the BL mapping seems to be okay. Do you have a xml or csv file that you use for the import, that I can use for testing?