I'm so sorry you are going through this. I don't know if it will make you feel any better, but I think all of us have had these thoughts before.
Wedding planning is tough, no matter how elaborate the wedding is!
I say step one is to take a deep breath. It's a Friday, so grab a cocktail after work. Have you addressed these issues with your FI? He might need to suggest to his mother that she back off a bit.
As long as the situation is addressed, and in an non-confontational manner, I don't see what the problem would be. Tell her you appreciate her help and enthusiasim, but you need her to run things past you first before she makes any decisions, etc. It might hurt her feelings now, but you don't want to start off your new life together with feelings of resentment for his family (trust me!) It will be better than completely losing it and screaming at her over the Thanksgiving dinner table.
Go on and mail the STDs. You will find that a lot of people will say they are coming and then don't. This is a whole different kind of stress. You just can't let yourself get worked up over hypothetical situations.
As far as who takes care of your son, you need to give each mom a schedule that you feel is fair.
Good luck! You're always free to vent here!