Is it permissible to increase the deferred Mahr (Sadaq) to guarantee the wife’s rights in the event of the husband’s death, given that the common fatwa in Iraq does not permit an increase beyond 19 mithqals of gold unless annual zakat becomes obligatory on the increase even before its possession?

The Shari'ah has not specified a particular amount for the mahr (dower). The default is that the man is bound by the mahr he named for the woman, and if they mutually agree on an increase or decrease, that is permissible. There is no zakat (definition) due on deferred mahr until it is received, and it is more cautious to pay zakat for one year upon its receipt. There is no harm in increasing the mahr if the husband agrees to it, but one should not request an increase unless he desires it himself.