Is it permissible to spend zakat funds collected for charitable projects (supplying water, building roofs, and constructing bathrooms) on other legitimate zakat disbursements, such as paying off debts and providing financial aid to the needy, given that disbursing zakat in its former form is impermissible and it is difficult to contact donors to obtain their approval for changing its destination?

Zakat is not to be spent on providing water and similar projects. Rather, it is to be spent on its legitimate recipients (masarif). If the zakat payers authorize you to spend it on something other than its legitimate recipients, you must refer back to them and inform them of this. If that is not possible, you are obligated to spend it on the legitimate recipients of zakat, because their obligation is not fulfilled by spending it on something other than its legitimate recipients. It is permissible to deviate from the instructions of the authorizing party (muwakkil) if they do not have a specific purpose in mind for allocating the expenditure, as stipulated by the jurists. Furthermore, it is not permissible to retain zakat and disburse it in installments except under specific conditions. It is advisable not to undertake the distribution of zakat unless one possesses the requisite religious knowledge for it.