Do I have the right to simultaneously study full-time for a master's degree at two different universities at the expense of individuals?

My son will be 16 years old in December 2023. Can we go to the military office for registration in the summer? Here they write that January-March is accepted for registration. That is, if we don't leave by March, they won't be taken into account already this year? And will there be a fine or something else?

If full-time studies are completed in June, for what period is the deferment granted? If I go with the documents to the TCC now, and update my data (I haven't updated it for a year). Will the postponement be until the end of my studies (until June) or a year from the date of updating the data in the TCC?

His brother is a military man who died during hostilities.

What documents should be presented to the TCC or to employees of the military office at block toasts? to have suspension from service? Is it possible to produce a certificate or document as a relative of a deceased serviceman?

If I am on sick leave, do I have to report to the military for a combat summons?

The serviceman fell ill with covid, pneumonia. He was treated for 8 days. He was taken out of state. Is this legal?