It sounds like you have decided to accept the visit, and you feel ready - as ready as you can be, protected as you can be. You will have this time with your parents with your eyes wide open, for the very first time. All truth, no doubts, no half-truths, no maybe/maybe not.

Your T is giving you this counsel as your professional caregiver, he's got some experience and he's trying to give you a sense of the impact that the encounter may have, even w/o any direct confrontation on the facts.

What will you do if violent anger appears? What about compassion? A mountain of fear and sadness? What about pity?

I know that there are plenty of people who don't even get a choice about having to see the bad person again, they are still living together. And I know what you mean about parents - it's a package deal. He's your bad guy and your dad. And I'm one who will never be face to face with the bad guy, ever, and I'll wonder what it's like.

I do know that when I've gotten "be careful" advice from my T, it's for a good reason.
I wish you a good outcome!

(p.s. You wrote that for Thurs, it will be "the entire day planned at something fun I wanted to do for months" - I really worry that it might be Dollywood, and you'll be disappointed! )