I can't answer all of you questions, because I only have one child and he is not of booster age yet.
I think that with the right combination of seats, you could fit 3 across, but I've never actually done it.
I've also not tried a booster in the 3rd row, but if it fit, I would be okay with it. The seatbelt buckle is kind of weird back there. Here is a picture. I would assume the position (and the fact that the stalk is not very long, may make it trick for a child to try to buckle their own booster.
The Frontier does fit nicely in the 3rd row with the long belt path, but there is no tether anchor back there.
The frontier also fits well in the 2nd row with LATCH or with the long belt path, but does not install in the center, because the seatbelt is too short for the long belt path, and the short belt path leaves too much movement.
I don't have a Nautilus, so I haven't tried any installs in the Pathy.