] In JA's books we often know the "fortune" which single young women have. (snip) I suppose some mismanaged and squandered, but was it the general case that such sums remained in tact both before and after marriage?

Under the law in JA's time and for 50+ years afterwards, a married woman's husband had the legal right to dispose of her property. (As Blackstone wrote, the law presumed husband and wife to be one person, and that person was the husband.) If she married without the proper legal arrangements being made, a young woman of means lost all control of her wealth and might have to sit helplessly by as her husband dissipated it.

Among the propertied classes, it was one principal purpose of the marriage settlement between the bride's father and the prospective groom to put limits on the husband's power, providing for trustees to hold the wife's property, specifying a minimum allowance to the wife for her own disposition ("pin money"), and providing for the payment of a capital sum or an allowance based on a capital sum on the husband's death ("jointure"). I don't know what man would make the arrangement on behalf of an adult woman without parents, like Mary Crawford or Caroline Bingley; presumably her eldest brother if there was one. As Persuasion points out, the widowed Lady Russell did not intend to give up her financial independence by remarriage.

The object of an unscrupulous man like Wickham was to get his hands directly on a rich young woman's money by marrying her without her father's knowledge and without a settlement. Lord Hardwicke's Marriage Act of 1753 was intended to and did make this kind of fortune hunting harder, by requring either a bishop's license or the public reading of the banns for 3 weeks in the parish church as a condition to a valid Christian wedding in England. (There was no civil marriage until the 1820s). Since Lord Hardwicke's Act did not apply in Scotland, the recourse of Wickham and his ilk would be to elope and marry there -- Gretna Green was the first town across the Border on the road to Edinburgh.