I'll try
So we have a 9 mon old, a 2 yo, a 4 yo and 2 6 yos? And a radian, SS1, built in, generations, titan, backless turbo.
The third row of a Venture is so tiny. Literally, so tiny. It really is hard to get 3 across. If you could find some old style parkways, it would be your best bet - and they fit petite children well. The armrests on the new one make it harder to buckle. Pw/radian/pw would work. You'd have to teach the kids to put their arm behind the pw wing to reach down for the buckle, then push the seatbelt into the buckle from the front. That's how my boys buckled when I had pw/ma/pw. Maybe that would work with the new pw too? It was harder for them with the turbo.
I read somewhere here that the Harmony Lite Rider fits in the third row, outboard, with a radian or marathon center. But that's a backless. Maybe the highback would too? Since you are in Canada, you'll need top tethers for harnessed seats, so that doesn't help. Even if it was your only option, it'd be better than no seat at all for them.
I could not get a radian to install rfing in the third row. I'll be getting one soon for my 3 yo, but it'll go in the middle row, passenger outboard. The middle seatbelts are tricky - but I've managed so far - pulling the base of the carseat just a little bit forward of the seat bight does help. I've only installed a ffing one a couple times though.
I would get 3 harnessed seats for the middle row. 2 rear facing, one forward facing. The forward facing one in the Latch position so you can slide it forward for access to the back. My 9 & 5 yo's have no problem sliding in that way.
So, a new rfing seat, maybe the new radian to 45#s passenger outboard, SS1 center, Maestro or Nautilus (or your generations if it still fits someone) drivers outboard for your 6 yo's full time seat and for whoever on the days you have all the kids.
Then in the third row, I'd try the new parkway/radian ffing/new parkway and see if the kids can buckle it. It'll be nearly impossible to climb back there to do it for them (unless you are like Gumby or something, I'm not
) If that doesn't fit, then the Harmony lite rider or Clek Oto if you can find some.
Good luck! I'm certainly glad I only have 5 in my van
There is a note in the Venture manual that they'll replace the buckles in the middle row under warranty. My dealer wouldn't because it's an 02, so it'd cost $75 + labor, but it might be worth it if you are having a lot of trouble - though w/2 rfing, 1 Latch, I think you should be okay.