Strictly speaking, since you're creating a center channel, I don't see why more speakers would be better. You seem to be alternating topics between midbass and L-R rear fill. Rear fill should have a higher frequency range than midbass so the "Cone of confusion" wouldn't apply.I was thinking about for the L-R surround channel. Using the 10w amps they had, I was looking for something with a ton of efficiency (which I erroneously thought wideband had), which would require a bunch of drivers and more than likely not be the best choice. Maybe something like the Tang Band W4-1805S might be more appropriate.
While my default preference is to mount NOTHING in the rear doors, I don't really have an opinion on what you should put there. I have been around the Speakerworks folks who have successfully used rear/side mounted midbass, and I've read the articles about multiple midbass locations along the 'cone of confusion' here. I have not had a lot (any?) of success with using rear side locations, but I think the problem is me and my lack of experience with digital time delay. That and I started doing this when such a device didn't even exist so your only choice was to get the path lengths right, which is still my primary choice.