Belt drive fixed

is it possible to buy the components separately ? i.e belt, chainring and cog ? if so what prices are they ? can't seem to find any reference to it at all.

Yes, but they are very expensive.
http://www.g-boxx.com/e-carbondrive.html
Swapping and changing gears is bloody expensive. Luckily I have 46:24 on the SS 29r, so that when I build an Alfine rear to make the bike more versatile. I only need the alfine compatible 24t cog (the only size they do)

It also sounds like theres limited gear selections mainly because you need a certain size of rear cog that cannot be too small so getting a gear high enough for the track might be impossible.

Currently 60:20 is the highest.

Yes, phil wood made a beltring out of metal, but look like it'll be quite heavy.

Just IMHO obviously, but I dont like the PhilWood design. It doesnt allow for clearance of small stones etc. from the sprocket/beltring.

is there no way the belt could slide off the chainring or cog over time? it looks like there
is no guard to keep it from going sideways. I am really tempted to get a trek soho with the
alfine hub.

The sprocket has a ridge on the left, whereas the beltring has a ridges on the right. The belt itself is so rigid that it is unlikely to twist off these.