Trainz 2012 will install along side of Trainz 2010 or any other Trainz product. if you own different versions you can have all of the installed and working. I recently just retired Trainz 2009, as now I only use Trainz 2010 and 2012.
Yes you can, though you may need to obtain any missing 2010 builtins from the DLS, which is where they are now located.
Or for those who have 2010 alternatively, save the ones you need from 2010 to cdp and add to TS12
Or, again if you have 2010 and you can't live without all the 2010 stuff just add all the 2010 ja files and 2010 CD key files to TS12.
I just added what I needed to keep the size down.