gilly, for your daughter's sake I think , armed with your knowledge about the bay, I'd write a nice little note (nice might be hard, but I'd do it with gritted teeth) to the aggressive neighbours and point out nicely that you have discovered from the management company that the bay is not theirs, and say the fracas was unnecessary, but you're sorry it happened. I'd also mention you are helping your daughter and are likely to be there again. You could finish on a light note and say you hope this has cleared up any hard feelings and you wish them well. (Yes, with steam coming out of your ears and teeth gritted so hard they might break!
)
It might help make your daughter feel a bit more secure too, knowing you have placated the nasty neighbours.
I'd also, in writing, let the management company know your daughter (with a baby) could do with that access to her house. I hope when the empty place is occupied, or even beforehand, a swap can be arranged.