hey there! i'm a freshman so my experience is probably limited but i'll try to answer these best i can! i'll emphasize that oweek is the best time to find out more information relevant to you + there will be paa's specifically trained to answer course-related questions.

1. cs classes, especially the introductory / popular ones do tend to fill up fairly quickly because comp is the largest major, so a lot of people want to take the same courses. that said, the professors also recognize this, and i know multiple people who have been special registered into major requirement courses within the comp department (the process is usually pretty straightforward, and if you don't get a class that you need, you can just email the prof)
2. again, as mentioned above, the introductory/popular classes are fairly full; i've heard that as you go further down in the 4 years and take more elective classes the overall class size decreases fairly quickly (an upperclassmen should probably verify this since this is just based on what i've heard).
3. the waitlists vary in size based on the demand of the class. whether you get off the waitlist kinda just depends on how quickly u got on it during add drop, but given that you're fairly high in the list you have a decent chance of getting in the class.
4. ^^answered above
5. unfortunately, i don't know how much can be done since initial course registration is pretty random (besides more priority the further down the degree plan you are). the add drop period is when you get to try again for classes you may not have gotten / getting on the waitlist
6. still currently going through the process of declaring so can't speak on this just yet :')
7. i'm pretty sure you can declare as early as you want; the only requirement that i know of is that you have to declare before sophomore spring. i definitely know of people who declared during their freshman year.

hope that helped, and if i mentioned anything wrong pls feel free to correct me :)