I think that maybe what Mary wants to do is to connect her branch of the big tree to another branch that already exists.
The term "merge" is used in a quite particular sense in WikiTree, for when you have duplicate profiles for the same person, that need to be made into one single profile.
If you need help with fitting your branch into an already existing family (which can be fiddly when you are new to the system) you had better mention the "endpoint" profile you have and which family you want to get it fitted into.