Below are the dimensions from the Polestar press fact sheet and a pic I took of our rear-facing seat centered on the rear bench. I confirmed for myself that with the rear facing seat on the center I could fit myself on either side snugly but comfortably. I'm confident I could fit 1x forward seat on either side and 1x rear facing in the center, maybe using the seatbelts for fixation and not LATCH/ISOFIX. I'm fairly confident you could fit 3x forward facing seats too, esp if using a Clek or Diono seat.
So I think your config of 2x car seats with one small adult or large child on the side should work.
The P*2 is sized similar to the XC40 and it fits three across in a variety of configs as indicated here:
A guide to which recommended car seats can be installed & fit safely 3 across in a Volvo XC40, w/ LATCH, infant, convertible, & booster seat reviews.
www.thecarcrashdetective.com
Of course, please check for yourself. At least the SF PopUp is OK with a crazy person doing all sort of unnatural things to their P*2 on display.
Edit: also note that the P*2's design there's a pretty wide space for shoulder area to the side of the rear bench. With the car seat in the middle I found I could use that for my shoulder width to fit comfortably - so the effective space is wider than what this dimension shows.
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