Alex,

For question 1. You can check on the United website couple of days or day before where you fly into. And even then, they could change on the day off.

2. If everything is perfect( no delays, in fact arriving early, arriving in C/D) that 50 minutes should be sufficient. Otherwise, I think there is a very good chance of you missing the flight. I would personally have at least a 1.5-2.5 hr layover at IAD from D-D or D-I. Which city are you flying to? Could you fly non-stop from EWR to that location? Otherwise, I would try to get an earlier flight so I feel better with the longer layover.