For a 7am departure, you need to leave the hotel no later than 5am and there is no MRT service to the airport then. So you will likely be taking a cab back -- and they do charge some sort of premium on the regular fare at that time in the morning.
I've taken the MRT to the airport and while you do have to transfer, it's not a big deal. The Orchard station is about a 3 minute walk from the hotel but not entirely sheltered if it is raining.