I have a 2012 Oddy and pretty much the same age kids. 7,6,4,2. We do 6 in the car on occasion.
So, the middle row--you can have anything there. I'd keep your rf kid there and put our harnessed Radian rider in the 3rd row. Do whatever you want with your two booster riders, but on the days you carpool, put the HBB rider in the 2nd row with the 8 year old in a low back booster. You can use any booster you want. There is tons of room in the middle of the 2012 odyssey.
The back is tricky because it is pretty small. We have a radian and a really large booster back (diono Cambria) there and can't fit any narrow LBB in between. I have a RiderSafe Travel Vest that is awesome for that middle spot. You could get one of those for your 5 year old carpooler.
I have not tried two narrow boosters with the Radian back there. Maybe a Dreamtime and a BubbleBum would fit. You'd have to try it out. I have to take my carseats out for someone who is working on the car on Wednesday.When I put them back in, I could put in the Radian and put the Harmony dreamtime next to it and see how much room there is left. I don't have two dreamtime though and the other booster I have (Cambria) is really huge so it woudn't be a good judge of how much room there is.
I have had 3 harnessed seats back there--two radians and a Coccoro, but the boosters take more room because you have to leave room to buckle.