@Zoobaloo I feel your pain because this was me.
My mum and dad divorced as my dad had many affairs and the last was the deal breaker, my mum never remarried. I was brought up by my mum and felt for her as she was the innocent party.
I wanted both my parents at the wedding, no-one but me wanted my Dad there and Dad wouldn't come without his wife (the reason my mum and Dad got divorced) and his wife stated she would come but wouldn't enjoy it. My sister said she wasn't coming if Dad came. I had invited everyone, with a carefully considered seating plan and order of service to avoid unnecessary conflict.
In the end I sent them all a message, its my wedding and I invite who I want. I will arrange things to make everyone as comfortable as possible, no top table etc, but basically this is my wedding and if you all love me, you will button it for a few hours for my sake and after that if you wish to leave early you can.
Sadly I can't give you a positive outcome that is relevant to you as my Dad died just before my wedding and my stepmum was unwilling to attend without him, which I understood, so the probably was solved.
You’re not responsible for protecting your mum from the consequences of your dad’s choices. You’re responsible for creating the wedding day you want.
If you want your dad there, invite him. If he won’t come without his wife, that’s his boundary and you can decide whether you’re willing to accommodate it. You invite him and if he decides not to come, that is on him.
You can manage the day so everyone is comfortable: no top table, thoughtful seating, separate arrival times, and maybe no FOB speech if you feel he cannot be trusted, etc.
But ultimately, it’s your wedding. The people who love you should be able to put their feelings aside for a few hours so you can have the day you deserve.
And its sounds easier said than done but I made the statement and my mum, please her, said she loved me and would do want needed to be done to make the day as happy as it could be for me. Now that is what you call a mother and someone that loves you.