This sounds like it could be an actionable offense… you might run into some issues though as there was a PVP solution to the griefing.
With that in mind, and assuming your story is 100% true, it seems that the PVP solution would be to just wipe the horde raid, so that combat on the boss resets.
It is not likely that Blizzard is going to compensate in the form of mounts. If anything, they’ll take appropriate actions against the accounts of the griefers. For that to happen, you’ll need to report them.