If a coupon has an expiration date, and the customer orders on the last day of the coupon but there is a proof delay and the order isn't actually submitted until several days later, does the coupon apply?

The coupon would still apply, yes. As long as they order on or before the expiration date :).

Thanks!

Another (non-hypothetical) question:

A customer ordered a photo and the order says "processing and printing"

Another customer ordered the same photo but has requested some further retouching.

If I replace the photo for the 2nd order while the first order is processing and printing, will I mess anything up, or is the file in the hands of Bay Photo now and changes on SmugMug won't matter?

I just answered this same question on the help desk 10 minutes ago, I think that was also you? Anyway, I'll post it here in case anyone else sees this thread.

Since this order is already processing that means Bay already has the images. You can go ahead and make edits to an image and replace it in a gallery without it affecting this order.

Yep, sorry, got impatient :)