However, there was on ditching incident (cant remember which now think it was one where the plane was hijacked) where people only survived because they HAD inflated their LJ inside the aircraft. As the aeroplane hit the water it borke up, flinging the pax in all directions. Because they had already inflated their LJs, when they ended up in water they floated...and survived.
However, I think 99.9% (if not 100%) of airlines say not to inflate your LJ inside the aircraft for the reasons stated above (which are more likely to happen that my example above)
And yes in the A320 you can use the rear doors in a ditching.