Is there the possibility of the orcs attempting only to free their comrades by trying to sneak into the pit...possibly by putting the tunnel that the Fellowship used to use again? Maybe Vror and the others didn't put the rocks back as well as they thought they had to cover it up? The orcs in the pit wouldn't be able to find it, and certainly not through the water, but the orcs outside might.
But again, Lindir could have been anticipating this, and they get caught as well in the pit. (They get into the pit to help free their companions and then a bunch of guards pop up above the pit, and there are some at the other end of the tunnel as well.) Then there isn't a direct confrontation, so neither any orcs or any of the fellowship will get harmed...?
Hope that made sense...
Basically what you're saying,
Child, just fleshed out a little differently...I think.