I absolutely was not suggesting Sir Thomas thought of bringing Fanny to MP to be a servant of any kind. (:D) Sir Thomas was not disinterested in taking Fanny in because according to the narrator he ‘had interest, which, from principle as well as pride—from a general wish of doing right, and a desire of seeing all that were connected with him in situations of respectability’ (1). This sort of pride is not a terrible degree of interest but it is there.

Darcy tells Lizzy he had interest, particularly her, in saving Lydia:

"If you will thank me," he replied, "let it be for yourself alone. That the wish of giving happiness to you might add force to the other inducements which led me on, I shall not attempt to deny… I believe I thought only of you." (58)

I am sure Darcy also thought if Lydia was completely ruined that Lizzy could be forever out of his reach. (:D) Mrs. Gardiner’s letter to Lizzy explains the other inducements which led Darcy on to save Lydia:

The motive professed was his conviction of its being owing to himself that Wickham's worthlessness had not been so well known as to make it impossible for any young woman of character to love or confide in him. …He called it… his duty to... endeavour to remedy an evil which had been brought on by himself. If he had another motive, I am sure it would never disgrace him. (52)