Mr Bennet's Grandfather proabably settled the estate on his eldest son for life(Mr Bennet's father ).

Then to his (Mr Bennets father's)eldest son(Mr Bennet) for life remainder to Mr Bennet's son.

In default the line had to go back to either Mr Bennet's younger brother -( if, indeed, one existed,or perhaps he was already dead and his decease pre-dated the start of the novel) and his male descendants, or , more likely, to Mr Bennet's uncle and his descendants (male of course).

Either way Mr Collins had to be descended from the male line and as such should have had the same surname, I am sure we all agree ;-)

To have a different surname means it was changed for some reason or another.

I think adoption by a rich non- relative or near relative, as in Edward Austen Knight's case is plausable ,not least because Jane Austen knew about all the legal ramifications of such an event, having surely overheard or taken part in conversations when her brother was " adopted" by his distant but richer relaton.