A lot of you seem to be forgetting to consider child/infant cart seats. Speaking as an adult with children, by far the biggest pain the butt with rear seats is dealing with car seats.
There is additional opportunity for innovation here beyond the swivel out seat idea that would be consistent with what we are seeing (although still highly speculative). For example, it might be possible that the 2nd row seats can tilt forward to face the floor to provide increased cargo room when needed, and if so it could do it without removing the infant car seats (in which case it should probably only be operable from the seat itself so that it would be next to impossible to do it with a baby in the seat).
Second, there are places where 2nd row cup holders could go, but most of the time, I would have child seats in the middle row, and if I needed some extra seats for adults or teenagers, they would sit in the 3rd row. Now hopefully they were able to give the 3rd row enough head room, but I wouldn't count on it. But either way, I'm not removing the infant car seat if I don't have to, so any extra adults or teenagers are going in the back. So cup holders in the back, good. Cup holders in the middle, don't care. Also, there might eventually be a middle seat console that can be used for extra storage and cup holders.
Third, it would be nice if there was a way to improve access for the 3rd row. If the 2nd row seats can tilt forward, that helps, but again you would have to tilt the entire seat because there will most likely be an infant seat there.
So the whole 2nd row not folding thing could be in no small part due to infant seats. You can't recline seats with infant seats in them, nor can you fold them, without removing the infant seat. Perhaps Tesla has integrated cool ways of making life with infant seats better, and that's reflected in the 2nd row design.
All of this is all just my own personal speculation based on what we see so far.
Dealing with children is a huge part of most adult's lives, and if Tesla is trying to cater the X to people with small children, it seems reasonable that finding ways to make hauling children easier would be high on the priority list for whatever magical miracle fairy dust seating they are trying to come up with.
PS: I'm not sold on the submarine idea, because it would likely conflict with the front seats when the front seats are reclined. There are solutions to that, like forcing the front seats to sit back up straight (reasonable considering the level of software integration), but there would likely be other obstacles like child toys on the floor that would make it very problematic.