A person could hold more than one living, and this was often the case. In fact JA's own father held two livings one in Steventon and another in Deane.

In chapter 30 there is an indication that Lady Catherine has more livings to donate.

"Very few days passed in which Mr. Collins did not walk to Rosings, and not many in which his wife did not think it necessary to go likewise; and till Elizabeth recollected that there might be other family livings to be disposed of, she could not understand the sacrifice of so many hours."

So, I would guess that Charlotte was really angling for any herself and her husband when hoping Darcy might help him in the church.

Was she realistic? Well I would say that given Darcy's character as it is revealed so far in novel, probably not!