Does a reply with both photos and text get counted in both categories?
No, it is either a photo reply or a text-only reply.
Does a post with more than one photo still count as one post?
Yes. I thought about making an entry for each reply and having metadata like number of photos and whether or not there was text, film type, camera type, etc., but it's a fully manual process. There is no Filmwasters api that I can get this data from (that I know of). I would have to download all of the replies and do some sort of indexing/searching/querying. I could probably write some R or Python code to do that. I have the interest, but not the time.