Alright, this clears things up quite a bit. I'm more concerned by seat selection for the transatlantic legs, on the way to the US that would be LH, same carrier as my first leg, meaning I'd get to check in and select seats using their check in, as I'm used to and on the way back, TATL leg is UA, same as first leg, which means I'd get to use UA check in and select seats from the non-preferred zone.

Thank you!