Direct Android OS update?

It's believed that Android product lines that run completely vanilla Android get their updates first because Google doesn't have to send them elsewhere to have third party UI changes added. This likely gets compounded by wireless carriers who want their bloatware added to the phones.

 

In theory, it is possible. However, you may have better luck finding information about your specific device in a more specific forum. Either in these forums Galaxy S2 section or Google search for 'Galaxy S2 forums'.

 

No...not really. If you want to do that you'll need to get a Google Nexus handset. They get the latest version of Android first.

 

Google only releases the source code. It then needs to be compiled for specific hardware. Since the drivers for the hardware are closed source, this becomes insanely difficult for people to do. There have been attempts, some successful (Cyanogen(Mod)) and some not so much.

You can't install clean Android on your phone like you could with a Windows laptop. The closes that you can do is root, install a custom recovery, and then flash a custom ROM that meets the criteria that you want. If you want something close to vanilla Android, check for Cyanogen(Mod) compatibility before purchasing a handset. Otherwise, you're mainly going to be stuck on custom ROMs that are variations of the OEM release.