1st- many people in USA have licenses for rifles and guns, why wouldn´t Joe?
2nd-
"The week prior to his death did not let me get closer to Michael.... What I did. I thought, I go home, take a gun at me and i open step. I told my wife: 'Katherine, we have to rescue the guy from there or will be dead in a week'. A week after he was dead"
It makes total sense to me. The only way that seemed plausible to take Michael out of that house was using and ambulance because he "died", other wise how could his family get close to him or visit him?
Looks like a very plausible plan!