JA has set up a situation which is enough to create some worry in me as a reader too; but the question is whether Mr. Bennet should worry, and how much. (I hope I am not beating a dead horse here.)

Mr. Bennet does exhibit some concern about Lydia's behavior, but he notes that Col. Foster is a responsible man. He is, after all, the leader of a group of militia that has been in Meryton for some time. (I agree that, if Mr. Bennet heard about the Chamberlayne incident, he ought to have noted it with more than amusement.)

Mr. Bennet also notes that Lydia is "luckily too poor to be an object of prey to anybody," so he has considered and dismissed the possibility of seduction by a fortune hunter.

It seems that Mr. Bennet, not overly concerned with how his family's behavior affects his own reputation in society, does not realize how dependent his daughters are on theirs. This is truly a flaw, one of many. But he seems to have answers to Lizzy's worries that satisfy him even if they do not satisfy us and do not completely reassure Lizzy.

I argue only that Mr. Bennet had a reasonable expectation that Lydia's trip to Brighton would cause the family no more damage than her staying home would.