Whether you agree with Catholicism or not (we know you don’t), we CAN show justification for Purgatory in the Bible, Mormons CANNOT show justification for baptism of the dead. The Catholic Church NEVER authorized the sale of indulgences, though it was done by some against Church teaching.
1 Corinthians 15:29
"Else what shall they do which are baptized for the dead, if the dead rise not at all? why are they then baptized for the dead?"
So, how about the justification for purgatory in the Bible? I've not heard it before.
It's a belief either way. What they are doing reflects on them and not on the person who has passed on.
The same is true if Catholics try to pray Jews who died in the Holocaust out of purgatory. The Catholics feel perfectly justified in doing so, but the Jews may well be upset about it.
If one does not believe that the Mormon baptism is valid, then ignore it. Let them waste their time and effort. At least if they are doing that, they aren't trying to convert live people.