Averil I think you are taking what you are calling "heirship bargain" out of context. Check out this link:
www.isle-of-man.com/manxnotebook/famhist/genealgy/cmpbook.htm
"Many of the leases were to endure for THREE LIVES. These were not three successive lives or generations, but rather existing lives chosen with a view to maximising the length of the lease, and the ensuring the security of the tenant's immediate family in the holding."