Yes, Mr. Bennet says Colonel Forster is a senseible man, but does he really know that? Or is he just assuming? And even if Col. Forster is sensible, will the presumably busy colonel of a regiment be the one who has the actually day-to-day responsibility for supervisiting Lydia? If he doesn't, it will be his wife, who seems more likely to egg Lydia on than restrain her, who will be "supervising" her.

Yes, Mr. Bennet has dismissed seduction by a fortune hunter, but fortune hunters are not the only men who prey on silly, poorly supervised young women. Just because Lydia is too poor to be an object of prey for her moeny doesn't mean she is too poor to be an object of prey.

If Mr. Bennet really doesn't recognize the importance of reputation in his society and how the ruin of Lydia's might affect the whole family, he must have lived his whole life in a cave -- and we know he hasn't.

IMHO, that we know about Wickham and Mr. Bennet doesn't is irrelevant. There are presumably thousands of young men at Brighton, and there are probably more than one that might take advantage of a silly young woman with insufficient supervision. Also, all Lizzy could have told Mr. Bennet about Wickham's bad behavior with women would have involved trying to marry them for their money. It wouldn't have made Wickham look good, but that could easily have been dismissed by pointing out Lydia's lack of fortune.

It's the fact that what seem so obviously specious answers (the above and others) satisfy Mr. Bennet that makes me think that he is either a lot less intelligent than he is usually given credit for being or he is being willfully blind to the dangers of sending Lydia to Brighton because that allows him to do what he wants to do.