You are not an idiot but your W knows you let everything slide so she does as she pleases. I am SO SO SO proud of you that you are putting an end to this. It's time.
IMO you are under NO obligation to share your legal plans and unless an attny is telling you otherwise I would not tell her about filing until you have filed. As a mere courtesy you may want to let her know you have filed and she can expect to be served on <insert date here>. I would, under NO circumstance, let her know the grounds you filed under, the monetary compensation or settlement you seek or anything about custody. Let her learn all of it when she is served.
Again, unless legal counsel is telling you otherwise I would split the finances before you do anything. You can simply tell your W that you have decided to handle your own finances and the start of a new year is the perfect time to do that. Work off that big list you posted the other day.
I would say nothing else about the OM. You said what you needed to say to her y'day on the phone. Give her NO further info and let her think the "old Kevin" will just tolerate it and go with HER plan. Again, this is just my opinion.
Do not accept anymore of her crumbs. If she attempts to explain herself tell her you are not interested in hearing about it. Let her know you will not tolerate or be a party to enabling her crappy behavior any longer.
Do research and find an attny that specializes in the rights of men. Read every review and public record you can find. Meet an attny armed with questions, facts and figures.
I would tell her nothing. Not a thing. You are not making threats, you are taking action and if she chooses not to behave as a W then you are under no obligation to tell her your plans.
