My comments:
A) Yes, you should absolutely avoid a PEK layover. Horrible experiences there. And in only an hour? No way.

B) If you are in EconPlus on an UA Dreamliner, you will have a more comfortable experience than Econ on SG 380

C) I would really urge you to consider: RDU/SFO, there are two non-stops on UA, and then SFO/SIN NS on 787. You would have a long, daytime layover in SF. But you might find that improved your experience. Take BART into the city. Have lunch at Pier 39 or Fisherman's wharf.

I realize that the morning arrival in SIN is irksome, but that's hard to avoid as it's on the other side of the world from the US east coast. And consider that on arrival, your body clock will be registering 'evening' during the Singapore morning. So if you can get into your hotel by 1pm, it won't be too bad. The east-bound itineraries (via LHR) will be about 25 hours from start to finish. That's not bad, but as you say the short layovers could be a problem. Especially early morning december weather into EWR (or even IAD). But if you fly west, spend the day in SF, and then on to Singapore, that long leg will only be 16 hours. And it will be at night, so you would more naturally sleep enroute.

Is this your first time in Singapore? It's a great destination.