Mail-in votes should favour Biden, page-14

To answer your last question first, I don't know, but I suspect all the preparatory steps (identification related, address etc) would not be as complex, as far as I understand it most counties don't even ask for ID, they simply take down the name of the voter and give them voting forms. I think (guessing) that even if they are not registered at that voting location they can register then and there and someone will assist them.

In other words, I think the rejection rates would not be as high.

To the first part of your post, here are the percentages for battleground states in 2016, as you can see these are very close margins. Maybe you think there's nothing to be concerned about but I think Democrat strategists would be thinking differently.