1st question: You can't the design of the discs themselves are different between bluray and hddvd. However, there are and will be many dual format readers. As for who would win, MS. MS is a software company with a little dabbling in hardware. They don't really care which one wins. Why do you think they have an external drive for the 360. They didn't want to put too much money into the HDDrive should it fail. They could just as easily, even said so themselves last E3, put out a bluray drive.
2nd question: Probably no-one. Sega's market share had already plummeted to the point they were not a threat to anyone when they left. If anything all have gained since, because they can now publish games on all systems. You could say sony, since most of the time they get Sega exclusives; or you could say Nintendo because of the VC exclusives.