It is very difficult to ban someone from Mass because they have sinned. In most cases it would take a restraining order and buy in from the Diocese.
So, as long as the other woman wants to go to Mass, she will likely be at Mass. I would not make this a hill to die on, I’d arrive early, sit on the front pew and not turn around to look at the other worshipers OR I would drive to another Parish. Does not matter who did what first to whom and for how long, I would simply remove myself from being further victimized.
It sounds as if there is some inside baseball here, Wandering Husband’s sister is friends with the other woman if I am reading correctly.
If they are all known members of the parish, and the parish has a policy where they allow members to use the Hall for baby showers, what would be the reason to forbid the hall rental, that there is sin involved? In that case, they would have to also stop allowing showers for children conceived in any sort of sin. That could be a whole can of worms.
It is hard to take a logical look when emotions are so high, but, my advice is to start Catholic counseling (I posted the Institute for Marital Healing above, call them on Monday!) They have years and years of experience helping Catholics through affairs.