The Olli looks the best right now. I'm guessing you have about 3-5 months before it fits more like the Vivo (which I wouldn't use) and you need the incognito.
It seems you have a really odd anchor geometry there though. Have you tried a different seating position? What about the Sienna, have you tried the third row there?
Thank you so much!
So, Vivo needs to go? Lapbelt too low? Am I understanding correctly?
Is the Incognito's lapbelt currently too high?
As for trying a different seating position:
That car is a 2014 Subaru Forester, it only has 3 seating positions in the back. DS is in a Frontier 90 (harnessed) behind the driver. DD is behind the passenger. We could switch them, but there is no realistic way to put one of them in the middle. The belts are mirror images on the two sides of the car, so I don't know that switching them would change any belt geometry, but I could try this weekend if you think it will help.
Sienna:
It is nothing but problems.
The incognito gave acceptable fit in the 3rd row, but we have to raise the headrest. That's fine if we are with DD/grandparents, but they won't leave the headrest raised when the kids aren't in the car, nor will they help her raise it when we're not with them and DD can't consistently raise it herself. We've tried to explain the importance of head support to them, but they don't get it. When grandma sits in the back, she never raises the headrest for herself either (which is why I almost always take the spot in the very back and insist grandma take the 2nd row seat when we all ride together).
The incognito is incompatible in the 2nd row; over 1/3 of it hangs off of the seat b/c of how the seat is designed.
The Olli gave terrible fit in the 3rd row (plus had the same headrest problem as the incognito), but looked like it gave good fit in the 2nd row. The belt geometry is different enough that the belt appeared to sit much higher on her pelvis than it does in our Forester. It looked like textbook fit. I just don't know how long it will last.
We took the back off the Turbo we had in there and tried it. It gave acceptable fit in both rows. In the 3rd row the lapbelt looked almost too low,
but looked still in the acceptable range (also still had the headrest problem). In the 2nd row, the belt was farther forward with it than the Olli. Not quite sure how that worked. ::shrug-shoulders::
Tried the Vivo too. Terrible fit in the 3rd row. Gave fit a little on the low side in the 2nd row. (But not as low as it is fitting in our car)
It seems like the Olli is the best bet in Grandma's car, but we don't want to buy a 2nd Olli.
We do have another cover for the Vivo and it is not expired. We can put the Vivo in the Sienna for DS (instead of giving him the Turbo w/SS) if you think we should put a backless Turbo somewhere for DD. The Sienna is the only place DS will be going into a booster.