1) I don't think there's necessarily a universal answer as far as whether all manifolds are expandable, but you can certainly pick one that is when you're shopping for one.
2) the manifold gets hooked up by connecting to the existing regulator's output, so no, you could not use something else on the output simultaneously unless you added a splitter, which makes less sense than just getting a manifold with one additional output.
3) I started with a simple Y and upgraded to a manifold, and I'm much happier with it. partly because I can now keep my tank outside the kegerator and just run a single line inside to the manifold, and the valves on the manifold are within easy reach to shut off kegs.
4) Kind of a vague question, depends on where you want it... but the setup you describe at the end sounds reasonable, provided you are also going to make an additional QD-to-corny-QD so that once you disconnect your tank/regulator, you can then connect it to a single keg.
on eBay, the user 'tw05' is selling some pretty cheap secondary regulators which you could use instead of a manifold, then you'd be able to control each keg's pressure individually. You'd need to buy some additional fittings to hook them up, and it'll wind up costing more than a manifold, but if you are thinking of upgrading to multiple regulators later on it might be worth it to bite the bullet and do it right from the start.