I believe the black coach is capable of bringing a Vampire back from the 'dead', although I'm not entirely sure how the process works. I thought I read somewhere that the vampire's remains need to be interred in the coffin, and the black coach needs to feed on blood to regenerate the Vampire.
One thing that could prove problematic is the way Ushoran was killed; according to GW fluff (unless they've changed it) he was killed by an Orc Sorcerer (I don't remember this off the top of my head, I had to re-read the fluff)
If you could argue that his supporters managed to gather his remains, and get him into a black coach, and keep that coach safe, I think you could be alright.
Alternatively, you could just say the army is Ushoran's army from when he was the ruler of his province. I know that presents difficulties with timelines, but I'm sure people could overlook them.
Weaponwise, I don't have an answer. The fluff I just re-read mentions "Ushoran had a unique skill with diplomacy and was a fine warrior whose lack of skill at swordsmanship was more than made up for by his immense muscular strength and finely honed strategic mind."
But that doesn't actually mention anything about a weapon, does it? It does mention him tearing the head off an Orc Chieftain with his bare hands but I'm guessing you've already read that bit.
So, I think the Coach could work, at a bit of a stretch. But I don't know about the weapons.
Hope this helps somewhat.