]Just wondering what would have happened if Mr Collins had taken it into his head to marry Lydia instead, and proposed to her - would Mrs Bennet have insisted she marry him - for the money and for Longbourne - even though she was her favourite child ?? :).

I would say yes. First of all, as Mrs. Bennet was so silly herself, she probably didn't see much wrong with Mr. Collins and more than anything, money/marriage was her utmost goal. Of course, Lydia would have refused also. Poor Mrs. Bennet, she was so sure that she had two thirds of her job done---two daughters married off and well situated fianancially and both opportunities have taken legs and walked off. ;-)