Following up on my question from a month ago about infant car seats in Polaris:
Our SFO-LIH flight ended up being on a 763 with the new Polaris seats. My 2yo daughter was in 3F (straight/forward-facing) and I was in 4L (angled). The FAs did not allow my daughter to sit in the car seat for takeoff or landing, but they did allow her to sit in it for the entire duration of the flight otherwise. They let me keep the car seat installed for landing in the empty seat (my daughter was in my lap); they didn't require me to remove it for landing from the empty seat and stow it in the closet, which would have been a hassle. The checkin and gate agents had no idea about the car seat rules onboard the aircraft, but the FAs were consistent and had definitive knowledge, and everyone was so nice.
On Diamond seats vs. new Polaris: I was worried that the new Polaris seats would be bad in this situation because there was an aisle separating my daughter and me. Actually, it was great! There was plenty of room for us to sit in one of the seats together for takeoff, landing, and meals. For most of the flight, it was a benefit that she was in a cozy, private seat without a lot of visual stimuli (kind of like how it would be in a car). She slept amazingly for most of the flight (much better than if she could see me because then she'd want to chat and play instead of napping). Obviously every kid is different, but in the future I would go so far as to say I'd even prefer new Polaris to Diamond even when traveling with a 2yo or similar age.