If you are going to take a taxi do you need to go all the way to Phaya Thai? Makkasan is nearer to the hotel
When I've taken the bus the driver has never cared about who is getting on. Many non-guests of the hotel just going to the shopping centre
The ticket for the airport train is a plastic disc and requires cash.
If you are American the easiest way to get cash may be to bring a $50 or $100 bill and go to one of the money changers next to the train station at the airport (Basement level). The rate is a lot worse at the shops elsewhere in the airport.
Sure you can get out at Makkasan and take a grab from there.
Dont need to take cash and use money changers. Right at the entrance to the ARL (where you buy the chip) are ATMs. Just get some money there. I find that having cash on you is a must in bangkok.