Good post, but some of this isn't quite correct:
On point 3:
a) Maths w/Physics is actually handled by the maths department and is more similar to IA Maths than IA natural sciences (so you have to sit STEP).
b) You can't switch to compsci from natural sciences anymore.
c) I'm not sure where you've got this from, but I'm pretty certain you can't switch subjects for part II, except to the Judge Business School. Also, part II is year 3 (&4), not 2&3
on point 4:
You're offered three (not four, though I might just be misunderstanding your wording here) of the courses you've listed plus one of maths or math bio