"what is the osmotic role of plasma albumin, and how would a low plasma albumin level promote edema"

Plasma albumins in the blood plasma are osmotically active solutes. They do not pass through the capillary walls to tissues as part of tissue fluid. Therefore they ensure that in the efferent capillaries, the tissue fluid flows back into the capillaries by osmosis. This is due to the fact that the blood plasma will be hypertonic to the tissue fluid.
Low plasma albumins levels will cause retention of tissue fluid, and it's excessive accumulation, in the tissues hence result to edema.