I think he may mean the 'clip' in the top of the lifter was missing; that would allow the lifter to pump up, push the valve into the piston, and bend the pushrod. If this happened, OP, the #3 intake valve could now be bent and would indeed cause the 0 compression and the intake backfiring.
As for #6, it certainly could be valve's badly sealing. Or it could be the rings. Go back and compression test #6 again. Then put 1/2 to 1 tsp of oil into #6 via the spark hole, and test compression again. If the numbers rise with the oil added, then you probably have a ring issue; if they stay the same with the oil, then look at the head/valve area.
Sounds like the heads are coming off regardless of where this goes. At that time, the cylinders can be inspected/ measured.
