Thanks so much for the info. So it seems that for TPE-BUS, the cheapest U fare is perfect as we get the seat selection in addition to the 100% mileage and a bag each.
For the domestic flight PUS-GMP, thanks for confirming about the bag being included. It just seems like such a cheap fare that I wasn't sure if it was like Basic Economy in the US, where no bags are included. KE seems a lot better as it includes one bag! We won't get miles but wouldn't get a lot on a 55 minute flight anyways.
Finally- for domestic flights, how far in advance should we book? 30-60 days out?