I am not a Glyptal fan. I never applied it, so I can't speak to that, but I have had two come in from respected builders, one which was a new Genesis block that came in with unrelated spun bearings.
That Genesis was flaking, but, when I decided I wanted it off, some came off with my nail, some with a pressure washer, some in a hot tank, and some wouldn't at all. It took forever to get to where I trusted putting it back together
As far as improved drain back, compared to a dry iron block, maybe, but once that block has oil on it, I would be surprised if it does anything. Just speculation though.
I can't comment on the sealing, but I always thought it was for electrical motors in some way. However, please never ask me to use it, and if it's on the engine, I am not very happy to hot tank and rebuild
