I will be flying UA from Bangkok to PHL and stay a week for a business meeting there. Then I will leave from PHL to DEN - since it's my personal trip so I need to buy this ticket seperately. And after staying another week in DEN - I will fly UA again back to Bangkok. In this case there shouldn't be any problem and Frontier should be a better choice I guess

By the way, after arriving Chicago, do you think 75 mins time is enough considering I have to get through customs as non-citizens, claim luggages and then jump to domestic terminal (which I have no idea how far from my arrival terminal) for my PHL flight?