Hello
1. Correct. You can enter the lucky draw the day before eg. today and will be told roughly by 7pm today if you get an appointment tomorrow.
2. You can't cancel appointment neither can you change the time. If the original appointment time is 3pm, the actual time may vary a lot as you need to refresh the page yourself very often (could be as early as 2pm or as late as 5pm or later, most often it's late than early)
3. No idea if this is true. I just know one you have an appointment already, your chance of having a second appointment is drastically lower (I had one appointment and tried consecutively for over a week without securing the second appointment to return the item.) also among everyone I spoke to, no one so far seems to be able to get a second appointment after the first.
There is no way to manage your spot. You are given a time that changes a lot during the day and you can take it or leave it. If you leave it, your chance of securing it again could be really low (from my personal experience, see my rant above, so please do take it with a pinch of salt)