I agree, in addition to the shoulder belt on the neck... I think this is a no-go.
Any other suggestions? So far, I've tried a Monterey, Parkway, Turbo, Turbo w/Safety Surround, and this Dreamtime.
Can you harness her? That seems like it might be easier in this case. Maybe a Radian?
No way will dh go along with spending that much to harness a 6-year-old (not that he's opposed to harnessing a kid that age...her twin brother is just barely moving into a booster...but he won't go along with buying a new seat just for that). Neither the Nautilus or the Frontier will fit in that space.
If there's no other likely options for the center seat, I think we're going to have to move on to plan B, which is put her outboard and deal with having to put the center seat up and down every time we load. Thus far, the Monterey, Parkway, and Turbo w/SS don't play well with the seatbelt in that location (although I called Diono yesterday and will hopefully have the new belt guide, which may solve that issue there).
A very last resort would be to put DS1 in that center seat in a backless. I'm not sure I'm ready for him to be in a backless yet, and I'm sure as heck not looking forward to pairing him next to the baby, but it is an option if worse comes to worse.