I see a lot of catastrophizing going on in your post, and I think that is probably understandable in view of it having just happened, but things are likely to be nowhere as bad as you fear and not as good as you had hoped.
Mediation is not normally binding. If you don't like what has been agreed to, talk to your lawyer about the chance to get something different from the court in view of the events that have transpired. Mediators do generally try to split the proverbial baby. I quickly looked at the statute (and practice there may deviate) but they can give more decision-making power to one parent and can change residential time if the circumstances warrant.
Keep in mind too that just because she may get half custody, does not mean that is what she will use. She may turn out to be flaky and take them only a fraction of the time. Three kids 9/10 would be very difficult for anyone to handle, even in the best of circumstances.
Also, try not to see the children meeting the bio parents as a bad thing. Adopted children always have those questions: who are they, what are they doing, why didn't they want me, are there other children. Sometimes seeing someone for yourself in all of their foibles is best. They will not romanticize what has transpired. Sounds like your kids do really see what is going on and really do understand who is the supportive parent. I think you just keep showing that through your actions.
You may find that you like having some time to yourself to recharge, heal from your ailments, work, seek entertainment, etc.
Document, document, document. If you find that things are as bad as you imagine, then move for a modification. As far as I know, child support and custody are always modifiable because it is understood that circumstances do change.
Try to focus on the positives. If you do resolve the issue through mediation, you save the time and money of court. You have reunited the children, which you've said was your goal. You've secured alone time with all the children. I assume there were also some resolutions on the financial side of things.
Now for a gentle 2x4. Still too much care going on about the W. She fired you. She is a mess. She is going to do what she is going to do. You can't change the outcome on your own. Only time and distance will tell you where this ends. Keep working on detachment and focusing on your, your kids, and that unit's happiness.
Now be proud of yourself for being an advocate for your kids and for fighting for what is right. Be happy you've made it through this stage. Enjoy them. The time goes by faster and faster the older they get.
