The Covenant get their resources from Forerunner artifacts that humanity was never able to translate because the Covenant kind of blew them up.
Superheated plasma wouldn't dissipate a ship, rather it would melt it.
If there are no Halo rings, the Covenant won't be present. They have no reason to be there, they are clearly outmatched, and they won't bother. Thus, in order for the battle to occur, the rings must be present.
The Covenant have no need to be present should the Great Journey not be possible. They'd keep a low profile, to say the least.
Yup. Completely forgot about that. The Halos still contain, y'know, the Flood though.
Regardless, they have a tendency to explode. A lot.