Hey, I just finished my first year reading law at Caius, lets see if I can be of assistance.
Congrats. Caius is a fantastic college and you're almost certainly going to have an amazing time. Worrying about things is natural at this stage.
Yep, at Caius there is the "MDR" (minimum dining requirement) which stipulates that you must pay for a certain number of meals per term (whether or not you actually go to them).
Lunch is optional, but on every day (except for weekends, where it is replaced by brunch).
For lunch it's a canteen system where you take a tray, get your food (served to you, just like your average canteen), then sit down.
For first/formal hall (the routine is the same for both) you sit down and all your food is brought to you. There's always a starter; main; and dessert. The only difference with formal hall is that you stand when the fellows walk in and there's a short Latin prayer before the starter is served (and obviously you wear the Caius gown).
None at all, you sit anywhere you like (or more frequently where there is a space or where you see a friend).
If you want to sit alone you can, but first hall is pretty full most days (formal less so, after the first few weeks the novelty wears off) so some people may come sit near you due to a lack of space.
But this isn't really anything to worry about, there's almost always someone you'll recognise or will recognise you.
Due to hall being all but mandatory you're unlikely to be sat with no one even close to you, so you won't stand out. In reality, people won't care who you're sat with. What do you mean by bother you? Some may ask to sit with you if they see you're alone and try to be friendly, but if you're opposed to that you could politely reject their attempts to be friendly?
Do you mean if you turned up to hall late or if you didn't go to hall frequently? Neither will 'count against' you in any way whatsoever (except that you'll be paying for meals you won't be getting - it's only compulsory to pay, they can't physically force you to go (nor do they attempt to)).
You will most definitely make friends during freshers week, I'm far from the most sociable of people and I shared some of your concerns before I started too but now I can look back and laugh at how worried I was for no good reason
I honestly think you're worrying about this way too much! Caius is a very friendly college, you're certainly not going to get judged or what-not for sitting at hall alone!
There's no dress-code per se, but headphones/hats aren't allowed. You're sat in a rather grand dining hall with 150-180 fellow students, you'd look a bit strange (not to mention impressively anti-social and a tad rude) wearing headphones.
By who!?
As I said above, the people at Caius (and at Cambridge as a whole) are overwhelmingly mature and friendly - it's not like school where people are mocked for certain things etc - we're all adults now.
I hope I've been able to help allay some of your concerns, if you have any more questions feel free to ask.