Nord I am so sorry your FIL was a violent abusive towards man towards your DH as a child. It is terrible that his actions caused so much hurt for your dh and his family.
I am glad your DH is now close to his mother and siblings.
As a Christian I am happy too that his father has found God and has worked hard to become a better person apparently. I wonder how you know all this, is it from your FIL via your dh or from independant report? Both the faith and the changed behaviour.
Re "DH had been really suffering with his mental health lately due to the past and the more he tells me the more furious I get at FIL."
It's totally reasonable for you to be angry with this man for the hurt he has caused your dh and by extension to your family.
RE "Aibu to refuse to see FIL or let my DC see him? DH had seemingly forgiven him." No, you are not wrong to do so. It is not an offense against you for you to forgive. If your dh chooses, once he has fully worked through all this, to forgive his father, you can choose to support him in this.
As far as I am concerned none of this means that you should feel you must meet this man or spend time with him or allow your children to be in his company. As others have said you must never let him be alone with them. And that does really mean you need to be with them when/if they see your FIL because otherwise you will place that 'burden' on your dh and I would personally feel that he will be dealing with enough coping with his own emotions.
Trifleorbust "...whether she should completely disregard her DH's clear wish to put it behind them" I don't see he has expressed that at all. In fact he is still being 'tortured'by the past so it is not behind him.
"I think that is unreasonable because she isn't worried he is a risk, she is just angry. It's not her place."
She may be at emotional risk, as may her dh and her children. And it totally is her place to care about her dh.
and
"But the OP clearly says her FIL doesn't behave like that anymore"
I am afraid the OP herself does not necessarily know how the FIL behaves. I doubt those outside the family knew what was going on when the FIL was being abusive to his family. It may be the FIL is not doing this now as the children are all grown up and his ex wife and he are divorced.
Or he may well have changed.
But even if he has stopped doing the cruel things he did in the past, it does not mean that those things done in the past are 'gone'! Those things have affected the OP's Dh and so are not gone and past.
There is a consequence for our actions and we pay that price for things we do.
As a Christian I do believe in forgiveness, and it is good to relieve ourselves of that burden to go on hating someone but even if we forgive someone it does not require us to have a relationship with them or put ourselves in the position of being hurt by them again.
Nord re "He's never met our newborn son but that is mainly because of the distance. I'm dreading Christmas as this is when we're due to see him and his wife, who both seem to dislike me, I'm assuming because I don't share their faith..."
Why are you seeing him at Christmas. I am afraid this is a very bad idea. Why are you planning this. IF you decide to meet him and let him meet your kids it should not be at Christmas because:
Christmas is a fun time for family and children not for sorting out very long age-old family issues and conflicts (IMHO)
You should be enjoying your new baby's first Christmas without this stress and drama
Your FIl can get to pretend he is a regular granddad meeting a new grandchild, but he is not! He has caused massive suffering and this cannot be swept under the carpet.
I am assuming your dh is seeing a counseller, please ask them if meeting the FIL at the most pressured, stressful, commercialized time of year is wise, (answer - it is not!) IMHO.