Is the fasting of a woman who felt wetness—without being sure if it was a discharge—after imagining her husband while she was making up a missed fast invalidated, and does she owe expiation if her fasting is invalidated?

The emission of mani (semen) or madhi (pre-seminal fluid) due to contemplation does not invalidate the fast according to the Hanafis, Shafi'is, and Hanbalis. They base this on the hadith (definition): "Indeed, Allah has forgiven my Ummah for whatever thoughts or notions they may have, so long as they do not act upon them or speak of them." The Malikis, however, differed, holding that their emission due to contemplation invalidates the fast. The preponderant opinion is that of the majority, and therefore your fast is valid. Nevertheless, a fasting person should avoid anything that arouses desire.