] On a previous discussion, don't remember where, the Longbourn entil was being discussed and someone said that if Mr Collins died before Mr Bennett the Longbourn Estate would go to his son, if he had one. Correct me if I am wrong, but wouldn't it go to Janes eldest son?

It goes through male lines, thus Mr Collins' son, not Jane's. We don't know how Collins is related to the Bennets, but even if it was through a maternal relative it's still an older line than Jane's. They're difficult, these entails, and work differently where I come from.