"With implied favourable qualification: Weather suitable for some purpose. Obs." (OED)

For a young, unmarried woman such as Marianne to correspond with an unmarried man was considered extremely improper. In fact, for a single woman to initiate such correspondence would be considered scandalous, thereby tarnishing her virtue and reputation as an eligible bridal candidate. Only if they were engaged to each other would this practice be looked upon favorably; in fact, it might be necessary and expected. If they were not engaged, and especially not to each other, open correspondence would indicate a lack of morals in the woman, in that society would assume the young woman to be the man's mistress. Brandon follows this statement by extrapolating that Marianne and Willoughby would soon be married, since that would be the only alternative so her social reputation would not suffer.

An exclamation, usually denoting intense agitation.

"Calling cards became popular at the end of the 18th century and bore the visitor's name, title and residence." (Grace, Maria. "Morning Calls and Formal Visits: Socializing in the Regency Era" English Historical Fiction Authors, http://englishhistoryauthors.blogspot.com/2013/08/morning-calls-and-formal-visits.html . Accessed 24 April 2017.) The purpose of the calling card that Willoughby leaves behind at Mrs. Jennings' residence serves to inform her, as the mistress of the house, of his presence in London. As her acquaintance, it would have been considered rude of him not to do so.