(1) the Scottish students won't all be a year younger. I've only met a handful of people here who didn't do 6th year [making them 16 or 17 when they start], there are a lot who are 18 when they start. I'm old for my school year, and turned 19 in the second semester of my first year being here.
Bear in mind that there are lots of people who take a year out or aren't Scottish [or both].
(2) If you A's in your Maths and Physics courses, doing direct entry shouldn't be a problem. About a third of people do direct entry, although that might not be quite the right fraction. The point is, a lot of people do it, you're not weird if you do it, and the university's very aware that you don't have a first year behind you already.
(3) My academic dad sings praises of how good first year was, and repeatedly encouraged me to not do direct entry. I liked direct entry anyway. However, doing 5 years probably gives you more opportunities to do interesting wee modules, and it would give you an easier lead into university life. The direct entry option was partially created [I think] to give a similar course to what English universities offer.
(4) The subjects you outlined would probably be fine, yes. The typical modules for direct entry to physics/astro are:
First semester
2nd level physics (30 credits)
1st level maths (20)
1st level astronomy (20)
Second semester
2nd level physics (30)
2nd level maths (30).
However, some people do some second level maths in the first semester, as well as the 1st level astronomy. You have to do maths stuff for 1st and 2nd level because it's so crucial for physics later on, but the maths lecturers are generally very aware that they're not lecturing a room filled with maths students, and I think that in MT2001 this year, they've got a handout on how thermodynamical stuff lead to fourier series [or something. I didn't get to read it]. From 3rd level, I think the maths you need is contained in physics modules, or maths modules specifically for physicists.
The other point with direct entry is that your astronomy modules would be as if you'd done first year entry. I don't think this restricts you, and they're very flexible later on with what year you do which modules. If you're only there for 4 years, you still get to do 5th level modules.
Another note on direct entry is that they'll give you some credits to make up the number required to graduate [or whatever]. So you don't need to worry about not getting credits from first year.
From what you've said, you'd be more than capable of direct entry. If you're doing further maths, then don't do maths and intro maths in first year. I did AH maths and didn't do any 1st level maths courses.
Sorry for the lack of structure =)
Edit: gosh, there were no replies when I started replying...