I also drive an 8 passenger Outlook and it's identical to the Traverse.
I agree with the sentiment that you're better off with a 7 passenger - in which case I'd go Acadia because the Acadia base model is 7 passenger while I believe you'd pay more to go to the 7 passenger in the Traverse..
There are shoulder belts in all seating positions, but the middle positions of each bench don't have a headrest. The headrests do stick forward somewhat, and are not meant to be removed, but I have been able to install most seats ff'ing in there. I've also had seats in the center of the 2nd row, some more easily than others, but I've only had 1 so far that was completely incompatible there. 2nd row has top tether anchors in all 3 seating positions, 3rd row just in the center position. I do find the center position of the 3rd row to be less than ideal for installing some seats because of the way the seat is contoured.
I've never tried 3 across in the 3rd row but have had 3 across in the 2nd row. With a radian in the middle of the 2nd row, you'd be able to choose from a good variety of seats for the other 2 seats in a 3 across. Of course ff'ing radian is hard in that position, and rf'ing radian is incompatible in that position, so that does cause some difficulty... but it's doable anyways.
We've been really happy with our Outlook overall. We have just over a year left on the lease, and will strongly consider getting an Acadia when the lease runs out on the Outlook. We will go with a 7 passenger next time though since it makes 3rd row access much easier if we ever want to use it regularly.