In article , Shawn
wrote:

What kind of vertical and horizontal room is there in the 3rd row?

Maybe 36" vertical and 9" from rear seat front to middle seat back.

Are these just child seats, or could smaller adults sit in them
upright?

The rear seat pans sit just above the floor, so anything bigger than an
8 year old will be cramped. Adults will be sitting with their chin on
their knees.

If one of the two were removed, is there a little room for luggage?

Yes