I found my drain tube plug unseated just like many people on here...this after ~1+ month of ownership. Luckily, despite parking it outside and it raining here in Seattle, no water managed to get into the cab.
Short answer to your ? of how to fold the rear seat back is:
-recline back seats to max
-fold up seat cushion (not all the way)
-reach hand behind the rear seat back and feel for a short pull cord
-pull said cord towards the closest window and rear seat back will unlatch allowing you to fold down (not all the way...seat belt strap will prevent this)... the cord does not pull straight down, you have to pull it at an angle
Why did I not get any water into my cab? As far as I can tell, there appears to be 2 parts of the rubber plug--one side that seals from the outside, and one that seals it from the inside.
After pulling down my rear seat backs, I found the inner seal was in place, and no water had penetrated. I didn't need to put any sealant/caulking in there...