Satan is the highest being in Hell. As Satan sits over the proceedings, some of the higher demons address the rest of Hell on the future of Hell and it's minions. Satan is speaks first to open the proceedings and speaks last to close them showing really how much authority he has in Hell. He attempts to set up the debate much like parliament would talk over a certain item, but it is understood that Satan, "the king", has final say and word over all.
The word except refers to Beelzebub as second in command in the hierarchy of Hell. Milton again shows us that the chaos of Pandaemonium is contradicting itself again by trying to apply structure to, what in everyway it's been described thus far, is really a structure less place, again showing the flawed nature of not only the demons of Hell, but the very place of Hell itself. Beelzebub is introduced to us as the second in command, the vice president of Hell. It is important for Milton to explain the demons in these positions to further exemplify the importance of showing a contradiction in Hell.
-Erik Scott