When you arrive DME, you have to go trough transit security, the funny thing the last time i went through 2 months ago , you can leave a computer in your bag , but have to take off your watch.
You can walk around in the departure hall,its pretty bad, not much to buy
just liquour, cigs and some dolls and thats about it.
Not sure about smoking zone as i donot smoke , so never really looked.
As to buying wine etc , I am not sure on USA rules, but I am pretty sure
they will not let you take it on as carry on anything over 100ml to a domestic flight you may have to put in in your checked luggage.Even though it was from duty free.
You should also check with Sq, about buying duty free in Moscow or singapore and taking on the flight from DME-Iah.
I have just flown from HKG-San Francisco, after security in HKG, my daughter had taken a bottle water from the SQ Lounge and when you board the flight
you go through another security check they check your carry on, on the jet way, and took her bottled water, even though it was after normal security, as they said that is rules from the the USA, nothing over 100ml, even if purchased after security, so I would check before you buy something and have it taken.
Good luck
