Must sumangali prarthanai be performed before the performance of upanayanam? We have not done this so far. Last year mother-in-law attained moksham as a sumangali, hence this doubt. If yes, what procedures must be followed by Srivaishnava households?

Sumangali prarthanai has not been prescribed as a part of Vedic rituals; nor has it been prescribed as part of ceremonies; if it is a practice of the respective household, it can be followed. Not performing does not accrue any malefic effects. If it is not being followed so far, no need to start any such new practices.