For cans which are a bit "sticky" in the valve, I've grabbed another rattle can of clear coat, removed the nozzle from each and pressed the two tube ends together. The clearcoat can shoots a small amount of wet paint/ solvent/ propellant into the stuck can valve assembly and seems to free things up fairly well - the small amount of clear coat doesn't seem to affect the colour of the other can. - the stuck can is on the bottom, the clearcoat can is inverted - so it's more propellant than paint being transferred.
for venting empty cans - I use the spike end of my welding chipping hammer - then slice open the can with tinsnips, retrieve the marble and dip tube and recycle the rest. The chipping hammer allows me to point the vent away from the body, and "hook the shot" with the hammer backwards so any vent starts away from me.
dip tubes are handy for making small cable guides, bushings, and other uses. marbles if cleaned on a shop rag within 20 seconds of the can venting are perfectly clean,and are used for bribing children, or thrown in a jar in the shed for floral arranging.