When are you planning on going? I think the smartest approach (I have done it a couple of different ways) is to wait until White River Campground opens and hike in over St Elmo's Pass, down onto the carbon and up onto the lower flanks of Ptarmigan Ridge. This way you can descend the Emmons and back to the car. White River road typically opens Memorial Day week end. I know for a fact the HWY 410 has already been plowed and the Sunrise road graded to the White River ranger station.
The road to Mowich typically does not open until late in the season, adding a couple hours to the approach via Spray Park (it took us 12 hours of hard hiking from the gate at Paul Peak trail head to the 10k camp). Going in via Spray Park necessitates leaving a second car at White River or Paradise or a long, tiring hike back around the mt, or a descent of Ptarmigan Ridge.
Have not doen Ipsut, but I understand the road is closed at some point, as you mentioned.