I'm so sorry you're having this experience.
Is it a 'missed' miscarriage, i.e. without any bleeding? Have you had hCG levels checked? If so, then yes, the ERPC can wait, unless there are any signs of infection (temperature, smelly discharge etc.). But you may want to consider whether you feel in any state to go through with the meeting. You and your physical and mental health come first.
Hospital may suggest you go for expectant management (i.e. wait to begin miscarrying) anyway. I live in a different country where they like to get ERPC done as soon as possible. I have accepted this in three of my four miscarriages for varying reasons. However, waiting to begin miscarrying and having a quick ERPC are all things considered about equal (although different) in risk - which is very low overall if you are keeping an eye on yourself, so don't worry.
An ERPC is usually a day case, yes. For mine I went in at 7am each time and was released between 2 and 4 the same afternoon. It is done under GA, though, so you will need someone to pick you up afterwards and won't be able to drive. After my first ERPC I felt well enough to walk (slowly and gently) home for an hour with dh and dc1 up a hill, whereas after my third one I spent the rest of the day knocked out on the sofa. In other words, the effect of the GA varies.
If you have the drugs you will probably begin bleeding fairly quickly and there may be pain. The bleeding will probably go on for several days (I've never had medical management so maybe someone who knows more can come along and correct me). I think medical management is safe for after a CS, as 'all' it does really (I think!) is induce the cervix to open and the uterus to contract - normal physiological processes, albeit for a very sad reason.
If you start to miscarry before Monday you should at the very least ring the hospital and see if they want you to go in. If the pain remains moderate and the bleeding manageable they may let you stay at home.
Again, I'm so so sorry. Miscarriage is a pile of shitty shite, and I'm not going to give you any of those pious lectures about being grateful you have a child. I have two and it was still beyond grim when I miscarried for the fourth time in total this summer. Perhaps, especially considering your history of ectopic, you might push for tests now? Despite not meeting the criteria for recurrent mc, my gynaecologist ordered tests after my last mc because of my history, and we discovered I have Factor V Leiden, which, fortunately, is treatable.
You'll be in my thoughts over the next few days.