No idea on the merc, as they didn't exist when I did it, but I wouldn't assume it is a problem because they "are a player". There is at least some precedent for "sometimes they are, sometimes they aren't". For example, I was recently trying to get the Valdoon raid to try to solo it for augs. You need 6 people to get those expeditions. However, mercs don't count. This is somewhat consistent with the fact that, like most raid instances, you can't bring your merc *into* the expedition, so sort of fair, but a real pain. I would also guess that mechanic wouldn't factor into the answer to your question. Not a lot of harm in trying, in any case.
And very annoying that an 85 paladin with FBoW and a level 76 ranger bot still couldn't kill the splitting mobs fast enough to stop them from splitting.
Picked up a weapon upgrade last weekend, along with an upgrade from Fero V to Fero VII, so I may try again, but not optimistic.