So, the current owner is going to retain title to the landlocked portion, but is going to lease it to the buyer of the homeplace tract.

Yeah, this will be great the first time a pissing match arises.

The owner of the landlocked part will nearly have carte blanche in court to devise an access route to his parcel. He will cite ease of construction, all weather access, etc, and any improvements the homeplace owner has made will be junior in nature to the access and subject to demolition, with or without compensation. (suck factor 9)

A route needs to be determined in advance, and the easement documents drawn up ahead of time.

Period.