Ooh car seat questions, I can do those :o
Pram compatability - this is totally personal preference. I really liked it but I didn't have it the first time around and didn't really miss it TBH except when I wanted to get a lift one way and a bus back or vice versa. It's useful if you'll be in and out of the car several times in one trip. Thinking about it, we do this more in Germany than we did in the UK (we live further from town here so often drive in for multiple errands). Bear in mind the car seat isn't ideal for them as they can't stretch out so you wouldn't want to use it all the time, you will need a lie flat pushchair or carrycot as well. Also, because a car seat is quite upright you have recommended time limits to avoid the risk of positional asphyxiation. 0-4 weeks (corrected for premature babies) up to 30 minutes in one stretch, 4 weeks+ up to 2 hours in one stretch. You only need to take them out for a few minutes but you should keep these time limits in mind. A lie flat seat does not negate the advice, as none of them are totally flat.
Yes, only infant carriers or car cots are pram compatible.
It doesn't matter if you start with an infant carrier or go straight to a from-birth seat which will last them longer :) But all infant carriers are designed from newborn whereas some of the combination seats aren't designed very well for newborns. (Joie 360 is fine unless your baby is very small.)
If you have a fixed seat you can just explain this to the hospital - they can't stop you from leaving! They may want you to use the pram to take the baby down for safety reasons or come to look at the car to check you have one, but in a lot of places they will just let you leave.
Most people get a separate seat when they are travelling so they can bring something more lightweight and cheaper in case it gets damaged or something which is plane seat compatible. Realistically, since we have no idea what's happening with travel, I would just wait and see how old baby is when you next book something and cross that bridge when you come to it.
I don't think the swivel function is useful at all with a newborn if you have an infant carrier. You would normally get them into the seat in the house and then take it to the car anyway. For a fixed from-birth seat, it's probably useful? To be honest I think the swivel system is most useful for the older baby/toddler stage!
The only difference between a 360 degree swivel vs 180 degree swivel is whether or not the seat allows you to forward face. So it depends if you're set on extended rear facing or whether you definitely want to forward face at some point before 18kg (about 4 years) or haven't made your mind up yet and want the option. FWIW it is much safer to rear face for the whole lifespan of the seat. The Joie spin seats don't have great leg room rear facing, which isn't a safety issue but bothers some people.
Joie i-Level is not a swivel seat, unless they've released a new one I'm not aware of? The original Joie i-Level works with the i-Base LX, which will take a toddler seat - Joie i-Quest. However this is not swivel either.
Joie are a good option apart from the leg room issue as said :)
Cybex Cloud Z is a gorgeous seat with great features, but personally I struggle with the pricing of it! Especially once you add the Sirona on as well - it's over £600 for the whole set. But if that's not a problem to you, go for it as it's defo one of the best options on the market right now.
The only thing I'd say about the modular systems with an infant seat + base + toddler seat is that they can be a false economy if you have a second child or a big/tall child. And a lot of times the cost of a modular system is higher than buying an infant seat with base then buying a toddler seat which doesn't need a base. The majority of them don't.
The only seat of the one you've mentioned that can be belt fitted is Cybex Cloud Z. The others are all isofix only, either with a built in isofix fitting or with the corresponding base.
Maxi Cosi are about to release in about 2-3 weeks a swivel set with an infant carrier and toddler seat that go onto a base which swivels. It's called their Family 360 set. So I'd hold tight for that as that may be worth a look as well.