Is zakat due on any of the aforementioned apartments whose payments have not yet been completed, and when does zakat become due on them, or is it only due on the rental value? And is zakat due on an apartment if its rental is stopped and it is put up for sale, and what is the mechanism for zakat in this case?

Zakat is not obligatory on real estate unless it is owned with the intention of selling it. However, if it is owned with the intention of renting it out, then Zakat is obligatory on its yield if it reaches the nisab (minimum threshold) and a full year has passed over it. Similarly, there is no Zakat on property owned for personal residence. Based on this, if the apartments were not owned with the intention of selling, then there is no Zakat on them. Rather, Zakat is obligatory on the rent of those apartments that are rented out, provided it reaches the nisab and a full year has passed over it.