I'm so glad for you that a way has been found that your DS is back home with you.
You are being incredibly strong in managing this in a civilized way and this should stand you in a really strong position for managing your way through to ultimately establishing independance for you with your son and you on your own.
Have you thought about timescales etc? When XH left i wrote a list that outlined the basic agreements we were putting in place at that stage. Like that the DS, in my case were going to stay with me, i would file for divorce and he wouldn't contest or counter file etc. We agreed the document both signed and then copied it so we each had a copy to work from. It wasn't legally binding as we didn't get witness signatures or take independant legal advice but morally both having it in writing as the basic outline it helped us focus.
I used email then and now as a polite way of communicating. You can write an angry aggressive email getting all your frustration out, then save it to drafts, calm down and edit it so you get your points across but edit out the emotion. Having everything in email format means that you have written record.
I'm guessing you have a rough timetable between you of when she will move out and the days between now and then you need to be kind to yourself. Sitting in stoney silence in the lounge isn't going to do either of you any good so things like going out to the cinema one evening, possibly offering for her to go out another will give you the breathing space to get through.
This is about long term civility and if you can maintain the upper ground morally, in the way you are now, it really will help smooth your future.
This weekend no doubt currently feels like a lifetime away and i wouldn't prejudge your religious bias but many businesses close for the four day period so I don't think i'm being to presumptious to assume that you and your sons mum will both be off work. Do you have family that maybe you and your DScould go and stay with for two days and then you sons mother have your DS the other two?
Having your DS overnight on your own is both good for your relationship and establishes that you can cope overnight on your own later down the track if his mother feels that she wants to be able to have it all. It may also be a true dawning for her for the situation created and provoke an interesting emotional response.
Keep strong.