So I got it and yay! It works!
When I first took it out of the box I thought no way this huge thing is going to work, but it's pretty much the right shape for the spot [bonus -- it's stable in the raised center seat, too]! And DD can buckle by herself, with a little effort though. Which is ok, because it's not meant to be her main seat for a year or two [every once in a while we need to transport another person besides me, DH, DD and DS, and with DD in a HBB it's just not possible]. DD does like the Connext a lot
The Connext seems roomier/wider than other boosters we have, which is good, because it does need to work up to until DD is 150cm.
Connext:
v.s. another one we have [sits weird and armrest nearly covers the buckle]:
I think the the adjuster strap makes the shoulder belt fit worse [more out on the shoulder], or is it just adjusted wrong? When DD buckles by herself, the strap kinda gets stuck too much to her side between her back and the backrest, which is why the diagonal belt stays further out. Any idea if it's safer with the strap but with the belt further from the neck or without the strap and the belt closer to the neck [not rubs-your-neck close]?
DD in the seat with and without the strap:
[Disclaimer: her jacket is not padded!]