One can certainly prostrate the elderly people, father-in-law, and mother-in-law at one’s in-law’s place even when they have not performed bharanyasam. It is a matter of regret for them to be associated with demigods. We can pray fervently to Emperuman to remove such tendencies. But one must respect the in-law’s position and must prostrate.
If people of other castes visit for Navaratri golu, we can certainly offer them betel leaf and areca nuts; and we can accept tAmbUlam from Iyers when we visit their house.
We can visit houses of other castes for auspicious happenings such as weddings, seemantham, etc. Otherwise, we should avoid.
If it is the practice of the household of one who has to perform bharanyasam to conduct Sumangali prarthani, it can be done, deeming it is being conducted for Lakshmi Devi.
If people of other caste have undergone samashrayanam, we must remember that they are Srivaishnava and must treat them with respect and must not commit an offense against them; partaking feast sitting together with them must be avoided; one must follow strictures of a caste where it must be followed. Similarly, there are stipulations that they must not see certain parts of our houses when they visit and we must ensure this is not violated. Otherwise, we must not look down upon them and must treat them with the utmost respect. We must be careful not to offend them.