What is the ruling on people gathering congregationally on Sundays and Wednesdays, and their fasting on Mondays and Thursdays, by order of the Imam who claimed it to be a Sunnah with baseless evidence?

Fasting on Mondays and Thursdays is legislated due to what is authentically reported about the Prophet, peace and blessings be upon him, being keen on fasting them. As for performing night prayers (qiyam) together on these two nights, there is no specific virtue reported from the Prophet, peace and blessings be upon him, for performing them congregationally or individually. Their performance falls under the general virtue of performing night prayers. However, singling out these two nights for congregational prayer is not legislated and is considered an added innovation (bid'ah idafiyyah). As for performing congregational prayer occasionally, without singling out a specific night, this is what the Sunnah has come with, as the Prophet, peace and and blessings be upon him, did with some of his companions. This applies outside of Ramadan. In Ramadan, congregational prayer is sunnah from its beginning to its end. The general principle is that voluntary prayers are better performed at home.