What is the legitimate Islamic way to safeguard the rights of electric elevator sharers, given that its cost was agreed to be paid in unequal installments, and to ensure the full amount due is completed should one of them die or sell their apartments?

There is no objection to writing the contract and clarifying its terms; in fact, it is necessary to guarantee rights and prevent disputes. The Sharia has encouraged writing contracts and documenting debts to safeguard rights from being lost, as stated in the Almighty's قول (statement): "O you who have believed, when you contract a debt for a specified term, write it down."