Ah... I just realized something. I use the car seat rear facing because... welll... that's what I'm used to. (In California, the law requires rear facing until age 2 or unless your baby is way off the charts big.)

So the reason the seat in front could not recline on Southwest was definitely because rear facing. As for what the seat didn't quite fit maybe it was also because rear facing - it's not unusually wide or anything. In fact, it's marketed as being great for travel (especially due to weight.)

Hm... maybe I should try front facing on this flight.

I'll actually have a car in the HK end (yeah I know its a bit weird) - which is why I'm bring this with me.