Assuming that i) Collins dies without issue before Bennet and ii) that this extinguishes the male line in which Longbourn was entailed, then the reversion of Longbourn goes to the heir at law of whoever it was who first granted Longbourn to the Bennet/Collins line in tail male. Depending on how many generations back Longbourn was entailed, it could be somebody completely off the map -- or it could be Mr. Bennet and his children.

Longbourn would not pass to Jane's oldest son. Where there was no son, daughters inherited equally as tenants in common, unless the owner made a different arrangement by will.

(Hope this isn't too technical.)