I missed this thread when it started, but had fun reading it.
If Johnson does decide to come out of retirement, he gets released immediately by the Lions. The Lions have about $8M in cap space and his $16M cap hit would kick in, giving them no choice but to release him. Johnson wouldn't come back to play for the Lions and I doubt he gives them a chance to decide where he goes, so basically he forces their hand to cut him by sending a letter to the NFL to be reinstated. Also, I doubt there are many, if any, teams willing to trade for a guy that has been out of the league for a year AND pick up his $16M contract.
Second. Why would the Packers even consider signing Johnson? As many pointed out, they are more than set at WR and the last thing they need is the distraction of trying to fit Megatron into their offense and getting his timing down with AR.
Third, the Packers have the money, but why spend it on a position of no need? If they were going to spend some money, beside on the obvious, extending current players contracts, the smart move would be on defense and trying to swing a deal for a quality CB or OLB.
If Megatron comes back, he won't be in GB and I am guessing he will be playing for less than the $16M the Lions would have had to pay him.