You are right Mr. Bennet could not change the entail but he was not helpless as father or a husband. He was not forced to neglect his daughters; he chose to neglect them because he did not wish to trouble himself. He was not forced to treat his wife without respect; he chose to because it was amusing to him. His favorite daughter tries to forget his behavior as she cannot justify it:

Elizabeth, however, had never been blind to the impropriety of her father's behaviour as a husband. She had always seen it with pain; but respecting his abilities, and grateful for his affectionate treatment of herself, she endeavoured to forget what she could not overlook, and to banish from her thoughts that continual breach of conjugal obligation and decorum which, in exposing his wife to the contempt of her own children, was so highly reprehensible. But she had never felt so strongly as now the disadvantages which must attend the children of so unsuitable a marriage, nor ever been so fully aware of the evils arising from so ill-judged a direction of talents; talents which, rightly used, might at least have preserved the respectability of his daughters, even if incapable of enlarging the mind of his wife. (42)

He is responsible for his youngest daughters’ misbehavior and how they got that way. If he had taken the pains to ensure they were properly educated and received proper instruction on decorum, manners and morals then he would bear less blame for their behavior including Lydia’s elopement than he does. If Mr. Bennet had saved money he could have increased his daughter’s dowries perhaps widening the number of gentlemen who might be able to consider them as a partner as well at being able to give them a better start in married life. This is in addition to his rather selfish regret after Lydia’s elopement:

Mr. Bennet had very often wished before this period of his life that, instead of spending his whole income, he had laid by an annual sum, for the better provision of his children, and of his wife, if she survived him. He now wished it more than ever. Had he done his duty in that respect, Lydia need not have been indebted to her uncle for whatever of honour or credit could now be purchased for her. The satisfaction of prevailing on one of the most worthless young men in Great Britain to be her husband might then have rested in its proper place. (50)

How do you know lack of visible concern (when concern was present) was typical in gentlemen of the time? Is there any evidence, aside from saving money, that Mr. Bennet regrets any of his other neglects? He does seem to lack empathy for Mrs. Bennet’s fears and while I agree she is rather dramatic at times I do not see how that excuses her husband’s ineffectual concerns for his family’s future. I don’t think Mr. Bennet’s neglects can be summed up to “that is how gentlemen were” instead of a lack of personal concern and responsibility.

What evidence is there in the text Mr. Bennet thought Mr. Collins would happily take responsibility for his daughters after his death? In Ch. 13 his remarks about Mr. Collins are sarcastic and it seems to me his reasons for allowing the visit are that he really had no polite reason to refuse and he hoped for some amusement:

"No, my dear; I think not. I have great hopes of finding him quite the reverse. There is a mixture of servility and self-importance in his letter, which promises well. I am impatient to see him." (13)

What evidence is there in the text that suggests Mr. Collins who advised Mr. Bennet leave Lydia to her fate is in fact a generous person or further that he would be generous at his own expense? Why would Mr. Collins rent out Longbourn house when by inheriting it he moves up the social ladder as a landed gentleman? Why would he continue to work as a clergyman and when he could hire someone to do it for him and still receive financial benefit from the living as well as maintain a tie to his patroness? Also Charlotte would surely wish to move back home and take her rightful position in society for her family’s benefit. Do we know Mrs. Bennet has fortune enough to rent Longbourn house—I am not sure that she does.