Is it permissible for the heirs to take their share of the "khulw al-rijl" (goodwill/key money) amount received by the uncle, given that it is a benefit resulting from the inherited lease contract?

The old lease contract, which has no specified duration and stipulates perpetuity, is Islamically invalid. There is no such thing as a perpetual lease, even if the law permits it. Therefore, the uncle or any other heirs of the grandfather have no right to a premium for vacating the premises.