Preserver didn't sound too imprecise to me, but I looked it up.

Samuel Johnson's 'Dictionary of the English Language', 1828 edition, gives the following:

To PRESERVE, v.a. To save; to defend from destruction or any evil; to keep; to season fruits and other vegetables with sugar and in other proper pickles: as, to preserve plums, walnuts, and cucumbers.

PRESERVE, n.s. Fruit preserved whole in sugar; land where game is preserved by the lord of the manor.

PRESERVER, n.s. One who preserves, one who keeps from ruin or mischief; one who makes preserves of fruit.

In the light of the above,I can think of two ways in which Margaret's description of Willoughby as “Marianne's preserver” lacks precision:-

None of the definitions given above carry the sense of “rescue”. Perhaps the imprecision lies in the fact that Marianne has already come to harm when Willoughby steps in, though arguably he is preserving her from further harm by carrying her home rather than leaving her out in the rain.