Is zakat obligatory on money that has reached the nisab but has not been disposed of for years due to compelling circumstances, and is this money considered "ḍimār" (lost/unusable) and thus exempt from zakat for the past period, according to the view of Imam Malik?

This money is not considered unrecoverable because it is a deposit in the bank and can be obtained. Unrecoverable money is that which is not expected to be acquired. Zakat is obligatory on this money for the past years, even if it were unrecoverable, the preponderant opinion is that zakat is due on it for the past years. Zakat does not diminish wealth; rather, it increases it and makes it blessed.