First, make sure that you really are going to be leaving Select. Amazon automatically renews you for another 90 days if you don't opt out. To opt out, go to the green "info" at the far right of your Bookshelf, click it, and it will tell you when your run in Select will end. There will be a box that you can check or uncheck to have yourself automatically renewed for 90 more days.
If you are not renewed in Select, you just stay in KDP. You'll be for sale just as you always were, except that Kindle Prime customers can no longer borrow your book for free. Others will be able to buy just as they've been able to buy it all along. No more free days, however.
No, you just leave it. Nothing happens to the book, just people can't borrow it and you can't put it on free promo. I always uncheck the automatic renewal box because I have loads of short stories up and I like to know when they finish a term because I can't be bothered to check. Therefore, they drop in and out of KDP Select all the time and it doesn't effect them on Amazon at all.
No, no, go to your bookshelf, on the far right you will find what you are looking for. 'you may have to move it over to see it' Just check or uncheck the box that puts you in the Select opition. You will still be on amazon, no need to do anything else. When the time is up you can then put it anywhere, but if you don't take it out of the select you can't.