1 and 2. This can very well be the case if when you're doing dumbbell rows, the line from your shoulders to your elbows makes something like a 90 degree angle with your body at the fully contracted position. Try to keep it around a 45 degree angle; what I mean is, keep it between a rear delt row and a dumbbell row when you have your elbow against your body.
3. Do you mean do rows with one hand first then the other vs. alternating them? (Or god forbid, doing them both at the same time?) The first is the best option. I say this since if you're trying to mimic a bent over barbell row, the way your arm is positioned and the way the weight tracks is going to move your center of mass around awkwardly, and you'd probably lose your balance if you aren't supporting yourself on something (a bench, a weight rack, your knee, whatever).