It is obvious from your first post that you take pride in doing things right
I would give her about 10 days for her milk to go if you are certain you can avoid her finding a tom or a tom finding her,remember she will start calling soon after her kittens go and Tom cats can force their way in to her.
As an entire queen she will technically be dominant to your existing cat but certainly wont be giving off smells like a tom cat would providing you do proper introductions all should be fine so dont worry.
I dont want to speak out of turn but it sounds as if this rescue is giving a poor deal. Kittens should be vaccinated at 9 weeks by the rescue not rehomed a week before this. Mum cat should be vaccinated, spayed and microchip inserted before rehoming. That said if this rescue is cutting corners here I am certain your girl will be better cared for with you on Sunday as planned.
Dont forget to checkup what worm/flea treatment has beenused and when.