What to use for family trip?

So we're taking a long weekend road trip with my parents in a few weeks. We've done this before with DH's parents (6 of us in our Saturn Outlook) but the kids were still harnessed then so I'm trying to figure out the best set-up for my booster kids

The kids will be in the 3rd row. This will just make things better for everyone. I get motion sickness even in the 2nd row (and I'll probably be driving some) and I'm not going to make my parents sit in the 3rd row for a 4 hour drive (they're both tall and 68 & 72 years old).

So the 3rd row has 3 shoulder belts and headrests outboard (forward tilting, non-adjustable).

My first thought was put both kids in their lite-riders back there, that's what we usually do when grandparents are with. But for a longer drive (and driving out will be after work, so later) DS still falls asleep often and is a wet noodle if he's in a NBB.

2nd thought was put DS's Vivo back there and let DD ride in a NBB. Problem is the headrests don't adjust and tilt, the Vivo says it should be flush on the back. I was trying to remember the thread back when Recaro started saying it needed head support and if they were the one that said it was OK to fill the gap with a towel or something (couldn't find the thread). So if DS uses the Vivo it's either outboard with a gap top to bottom at the back and DD outboard on the other side in a NBB. Or the Vivo actually fits nicely in the center 3rd row (and the seat back is tall enough to support DS's head properly), but DD would need to use the Bubble bum to fit next to it. And I'm not sure how both kids would do sitting that close to each other.

Last option is having DS use the FR80 as a booster. It actually fits nicely (tough to buckle but almost anything is back there). Then DD could use either her lite-rider or NBB PW. My one hesitation is that it's a heavy seat and it's not latched or tethered. But I think it might be the best solution.

So WWYD?

I'd use the Frontier either harnessed or as a booster, whichever you want. I'd rather use a booster correctly (Frontier) even if its heavy and not latched than incorrectly (vivo with gap).

For a 4 hour drive, I'd prefer high backs for both. My kids don't do as well in NBBs in our 3rd row since the seat back is so flat. The Captain's chairs are more supportive it seems. For anything over an hour, even my 9 year olds opts for high back. Actually, she opted for harnessed in FR 85 last week on a 10 hour trip. I'd check with Recaro to see how to safely use it and ask both kids which they prefer.

I'd use whatever was used properly and the kids like best. Realized on a long drive recently that my 7 year old would probably be more comfortable without the back on his nautilus. I am likely going to buy him a new comfy LBB soon.

To clarify - the Vivo fits flush if it's in the center, but then DD would have to use the BB to fit next to it.

I'm not worried about her, the back seat is actually as comfy as the 2nd row as long as you're not a large person (contoured seats), and there are side airbags back there also. She's not a car sleeper unless it's a really long day of driving (like 6-8 hours).

I was leaning toward the FR for DS. It surprised me that that huge seat actually fit the spot the best.