From: Rabbi Moshe Sofer
To: Unknown recipient
A Kohen whose wife died before purification following a chuppas niddah is not obligated to become tamei for her and should not observe mourning. Since chuppas niddah creates inheritance rights only through mechilah rather than rabbinic enactment, the meis mitzvah reasoning does not apply, and the principle of shev v'al ta'aseh requires the Kohen to refrain from tumah; mourning is similarly prohibited lest it lead to erroneous tumah.
Source: Shu"t Chasam Sofer EH I §121