Generally, in society widows are not allowed to participate in auspicious functions. Why are they being avoided? Are these new rules brought in by us or are these scriptural ordains? Are these accepted by our preceptors? Is this applicable for Srivaishnava women who had done Prapatti? If those who have done Prapatti are Nityasumangalis, then what is the harm in inviting them for auspicious functions? Please pardon this query if it is inappropriate.

This should be understood from the standpoint of our Sampradayam. Couples are given importance in Grihastashrama and they are considered distinguished. As per the ordains of scriptures and Sampradayam, kainkaryam has to be done as couples. A single man or woman is not given importance.
The husband loses the eligibility of doing certain Vaidika karmas if the wife is not alive; similarly, women are prevented to participate in auspicious functions. Our tradition has considered couples participation as ideal.
It is appropriate for a Prappana to realize that Emperuman is the only Paramapurusan and all of us are the ladies. There is no barrier in rendering services to Emperuman. A single woman can still go to temples, recite Stotra-s, Pasurams and render kainkaryam to their ability.
Scriptures and Sampradayam have laid down prescribed rules for certain tasks. Even people of this world follow the rules that are applicable.
For e.g., like how widowers are not eligible to perform Yagyam, bachelors are not eligible to perform aupasana and a question shall not arise why because only married men are eligible. Similarly, there are prescribed rules for kainkaryam based on the varnashrama. Elders have been practising the ordained rules and it is best for us to do the same. Engaging in sampradayam as a couple is considered appropriate even from the perspective of Acarya.