] That seems to say that Mrs. Bennet did have the faculties to properly evaluate Jane's condition.

The apothecary, Mr Jones, seconded Mrs Bennets wish that Jane stayed at Neatherfield. Jane seems to stay in bed almost two days after the visit so Mrs Bennets decicion to leave her there might have been out of concern about her daughter too. That fits the character of both Mrs Bennet and Elizabeth, though their reasons may have very different background.