I believe like lot of other individuals, Taiji and Tauji also had their own initial reason to oppose the marriage. For Tauji caste & related social dogma was supreme, which he was ready go to any length to uphold. Although we are hating on him, it was common to find unrelenting people like him in the 90s. For Taiji I think her dislike for Rakesh is the foremost reason of the opposing Naina marriage. Whatever the earlier reasons were, it has been overtaken by ego now. Even if they are willing to give up their stubbornness in their hearts, they might not be ready to do so in their heads due to ego issues.
Inside every person, there are two opposing forces that constantly conflict each other one is love and the other one is ego. Tauji/Taiji I am sure do love their family and deep in the hearts are probably troubled by the opposition that they have put up. However, under given circumstances, they have let their ego rule over themselves which is making them reactive and desperate to gather their relatives to be their allies so that they can validate themselves as being right. Eventually Taiju/Tauji will have to choose which force will govern their decisions - will it be EGO or LOVE for their family.
Should they participate in the wedding? My answer is Yes, they should participate. They are the elders of the family and their blessing will mean a lot to the people involved, namely Tauji's two brothers Anand and Rakesh.
Will they participate? Yes, I believe that they will participate in the wedding in what capacity I don't know but I do believe that they will participate.