...may have been commoner in JA's time than we now think. As you say, social circles were more restricted then, and Charles Musgrove doesn't strike me as the kind of man who would willingly spend much time in a social centre like Bath looking for a wife (though I may be wrong about that). If there was a reasonably attractive local girl who liked him, why not marry her?

I know it was frowned on for a man to marry his dead *wife's* sister, but before marriage I think it was considered OK to switch from one to the other. Think of Elizabeth and Wickham in P&P. As far as Wickham knows when he arrives at Longbourn as Lydia's husband, Elizabeth may still have feelings for him, but now both of them have to act as if they were only "always good friends".