They were suffering from lead poisoning on top of the scurvy. There was lead in the cans... Which they had been ingesting for years. It wouldn't have been so much of a problem if they had been on the mission for a year or so...because they would back to eating non-canned food. But unfortunately they were stuck for several years, so it was a constant ingestion. Awful really.
"Boys" were the lowest rank on ships - they were "green"...the higher you went in rank, the different your punishment was. Has Hickey not be insubordinate, he would have been lashed on the back, as the other two were. But he was insubordinate, so he received punishment fit for someone lower than his station.