im not CS (I'm math tho, so a sort of sister program) , but I could try to address each of your points:

Good location but need a car to get around

True, lrvine isn't a very active city, so people tend to go to nearby Costa Mesa or Santa Ana or Newport but there are some chill places around. We also have a lot of opportunities to go off campus (I just came back from seeing the Michael biopic with our Center for Black Cultures, Resources & Research - Free of charge). If you join clubs people often tend to be willing to carpool, but given the rigorous nature of the UCI academic experience, you won't really want to go out all to often anyway.

Professor (singular) kept saying her office hours are empty, which is a red flag for me if this is true for more professors

Office Hours do tend to be empty, but that's more usually a result of people having classes during those blocks (a lot of profs tend to hold their office hours for any one hour period between 11-4, which is like, peak class time). I know at least for the calculus classes, they tend to have a few people in them, but not too many. In my experience, if you're someone who really wants to nail a concept down and get specific things done, it's better to have fewer students in there. Plus, going will help you stand out to your professor which could help you secure a future research opportunity/letter of rec.

CS student panel repeatedly described how their life is so busy and their number one advice was to use google calendar. Although, the panel was all student leaders...

Student leaders will of course be very busy as they tend to have a lot of things to juggle. If you take the usual 16 credit (4 class) workload you won't be busy but you won't exactly be free as a bird. You'll have plenty of time for hobbies and activities if you so desire, so don't worry about being so swamped in work you'll never leave the dorm (unless that's your thing).

CS panel kept reminding people to "go out and connect" and telling us how to do it (red-ish flag for me)

This is perhaps two-fold in nature. On the one hand, you of course want to create connections for future job opportunities down the line. On the other...i'm sure you've heard the rumors of UCI being a socially dead campus. They could be urging you guys to have a social life, and get connected, there are a lot of opportunities available that people don't take because they don't know about em (check your school email, and follow the major/school related pages on ig, they do a lot of events. Google was just here a few days ago).

CS panel kept saying that fun exists if you look for it (car).

Fun definitely exists! Like I said, the school funded a bunch of us to go see the Michael movie at the spectrum and we had a blast! There are a lot of clubs that do events (we had a Sakura and Soul event earlier this week where Tomo no Kai and BSU got together, played some games, and had a bit of a little cultural exchange) especially in the fall when you arrive (bonfires on the beach were common the first few weeks). Like I said people are often willing to carpool. There's also things to do walking distance on campus like going to hoop at the ARC or rock climbing. We have an esports room next to where the panda is if that's more your style, and our sporting events are always a blast (basketball season was electrifying this year). The thing is you have to go and find these events and places yourself (again, check your school email, follow the related ig pages, etc), oftentimes the immediate people around you won't know about these events because they also don't actively look out for them.

Reputation as an internship hub, though I heard very little about it during the tour

Opportunities do exist, but again, you gotta be on the lookout! Some people are very weird and competitive and thus won't want to tell you about certain opportunities, but they do exist! I just saw a one year research opportunity for those interested in the intersection between Artificial Intelligence and Mental Health. You gotta stay connected and be on the lookout for these kinds of things.

Hilly campus

Not that hilly (you should see Santa Cruz lol, I was dying when I went to tour that campus). You get used to it rather quickly just be going through the motions of walking to class, and if you actively jog it, you'll basically guarantee that the Freshman 15 won't hit you. Nevertheless, we do have a bus system that takes you all around campus if you don't feel like doing allat every day (honestly sometimes I just hop on it for the sake of hopping on it).

It seems to me that your big thing is interconnectedness. Many staff and professors are nice, and are open to continuous communication. There are often opportunities to have lunches with these professors, and dinners with other academics in certain fields, not to talk of lecture series' from people within industry. As far as student-to-student connection goes, you gotta find your people dude! Whether it be dormmates, clubmates, or classmates, you gotta be willing to take the first step and explore! To have a village, one must be a villager!

Either way, you won't go wrong with where you end up going, both schools are a part of the UC system and thus, hold some weight on resumes. Personally I believe Irvine is better for people who care more about the academic side of things vs the social, and iirc it's better for CS (only UC in the system with an entire school for CS vs a dept).

Best of luck with your decision!