The first working armor was the Mark V I believe, as seen in Halo 1.
Halo Wars is supposed to take place before Halo 1, around the first encounters between Covenant and Human.
(The first humans who encountered a Covenant [a Jackal I think,] were three Spartans, John and his two friends Sam and...Kelly? aboard a Covenant ship.)
Anyways, the armor that we saw in the trailer wasn't Mark V or VI...so, I don't know.
And I agree with you about the first two things.
I'm pretty sure the first humans that actually lived to encounter the Covenant were Sergeant Johnson and his old friend Sergeant Byrne in the book Contact Harvest on board a fruit freighter set as a trap. But the first species in the Covenant that was encountered was a Jackal.