Homsap:
a) No, tailstrike not prevented by software. On latter models an indicator is on the PFD to show limits and a 'Pitch Pitch' aural warning from the FWC. Edit: There was a ELAC update a few years back that 'helped' avoid but wouldn't prevent tailstrike during landing in case of gross mishandling.
b) It is standard, there is little difference between 319/320/321 (never flown the 318) The 319/320 pretty much the same. 321 flies like a bigger aircraft but pitch the same
c) Indeed, it would be very very rare to hit the backstop of the sidestick controller. We are never really meant to hit the limits, but I do on roll on occasion. On pitch, I can think of 2 events in my time on it.
It is no issue flying the different variants unless you mishandle the aircraft to an extreme.