The elevation doesn't clarify it but I would say that if at least half of a big model is elevated, it's all counting as elevated. I'm seeing it rationally as that unit just balancing on a tight spot for its size
The only units which are thralls are the ones with the trait. All others get just one unit.
The virulence does seem strong but maybe it's balanced out by the randomness? It not that much stronger than the carcass or goregrinder, but it might be balanced around the whole faction instead of only between necromancers.