I was able to answer one part of my question above--that is, it is totally okay to use vertical images. The images you upload do not have to themselves be 4:3 or 400x300 pixels. However, whether all images appear uniform size on auction page depends on whether you save them all to the same size before uploading to Auctiva. At least I think that's correct, having tested some more.
But the rest I'm still not sure about.
Thanks again.