i believe you play as covenant in halo 2 because i think they wanted to give you something different in it and the story wouldn't really make sense if you didnt play as them.
Probably not ONLY covenant, but Halo 2 already has about half of its levels where you play in the life of the Arbiter, one of the Elites shamed after losing the first Halo ring. Both Halo 2 and Halo 3 allow you to play as an Elite in Multiplayer, and Halo 3's co-op allows you to play as the Arbiter for the second player. I hope this answered your question.
You cannot play as Covenant in the original Halo
NOPE
Yes, in Halo Wars you will be able to play as the Covenant. I dont know, however, if you will be able to play a story mode as the Covenant, but I know that you will be able to in Free Play and Xbox Live.
You build a factory
no
In Halo 3 the covenant does die, the covenant will never be in another Halo game.
Not exactly. Instead of a Halo "4", Bungie have decided to release Halo:ODST. This game bridges the gap between Halo 2 and Halo 3. You play as an ODST marine in the game. Also, for RTS fans, Bungie have also made Halo Wars, a strategy version of Halo, in which you can play as either the marines or the covenant.
The covenant is an alien species in halo including grunts and elites.
Halo 2 is mostly set through out the game based on another Halo ring where the covenant have moved to. It is also set at the start of the game when you can pick up your controller and play as master chief on another human ship with different soldiers to fight against the covenant and stop them from boarding and planting a bomb to blow it up. Master Chief then visits Earth to stop the covenant there and then chases them to another halo ring to investigate. You play as the arbiter where he has to defeat the heretic leader and hold back the flood visiting two ships then he visits the ring to stop the brutes.
If You Mean change into elite, no, you can only do that in Halo 2, 3 and Halo Reach. But however you can use mods at halomods.com
Maybe. with alotta money mind ya