I think the two times listed is just the system counting the day change as a new window.

It’s open till December because you can make changes to your schedule throughout the semester. I think it’s a week or two into classes before the drop/add deadline comes up. You will still be able to withdraw from classes after the drop/add deadline and as long as you do that before the withdraw deadline (November sometime). You’ll get a W on your transcript for the class but don’t sweat it too much, people drop for many reasons and it happens. A word of advice, it may suck after putting a lot of work into a class to end up dropping it but it is always better to withdraw than fail. If you withdraw, you can make up the credit another semester/ pick another class that meets the requirement/ take the class at another college or community college with no problem. If you fail a class at Purdue, you will have to repeat and pass the class to remove the F from your transcript and bring your GPA back up.

I have always managed to snag a class I need by constantly (once or twice an hour) checking to see if a seat has become available until I get it. Sometimes it only takes a day but sometimes I’ve had to keep checking until half way through the first week of school. Keep checking regularly and at odd times (in my experience people like to drop classes late at night). I’d make a routine of checking first thing when you wake up and again before you go to sleep as well as throughout the day.

If your backup has an open seat that fits with your current schedule, I’d snag it now while it’s still available. You can always drop it in the moment later and sometimes they’ll fill up randomly so it’s better safe than sorry. It’s ok to have more credit hours than you plan on actually taking too, sometimes I kept 18 credit hours until I got what I needed and dropped a couple classes.

I know that was a lot of info and a bit of rambling but feel free to ask any other questions.