Title is pretty self explanatory, Can the commissioner of a piece of art be used as the primary source for a post? I might put the commissioner's link at the bottom of the source list but commissioners don't always link to the artist of the piece so people could think they were the artist

I would say only use that if there is no other option (artist does not have it on their own page for whatever reason). But I'm not completely sure. Either way it's not bad to have as a backup source in case the artist switches accounts or something.

If it's available on the commissioner's page and there are still spots left then definitely add it in. Many valid sources are more useful then too few.

Yes, though artist is preferred. I always try to put the artist's source and the commissioner(s) source when I can, for example post #1346102.