"It's a shame that your DH and his Mum can't repair their relationship, though."
It doesn't sound as though they had a loving parent/child relationship to repair.
Unless the neglectful/abusive parent is aware and aknowledges what they have done, then it cannot start to be put on an even keel.
I also don't think that a cruel/bad parent should necessarily be forgiven, it is up to the adult child to decide.
OP, I think that you should try to avoid having to aknowledging her, by not looking at her. It us reasonable to do this at a funeral.
How would your DH feel if you smile at her? I would nod but not smile.
I wouldn't try to be "the peacemaker" anymore.
I often wonder why, just because the child has managed to go in to build a life, that others think a person who has been a awful parent, should have allowances made.
Not putting yourself in the company of your parent, so the abuse can continue, can be so liberating.