Interesting how we project missing information onto the stories we are told. When the OP described the problem with their vocalist/guitarist, I assumed that the issue was making the vocal part work, especially with typical crap monitoring, while you seem to assume that they're having problems with learning the guitar part. Problems with the guitar part could indeed be down to lack of study/practice, but as an occasional vocalist myself, sometimes the bass note resonating in my chest helps me find and stay on my pitch when I can't hear myself properly in the monitors. OP mentioned that they had to change the key for the vocalists range, which suggests that the value of learning from the original recording is limited (I have the tech to transpose recordings, but I've been a hacker for almost as long as I've been a musician). Could be both of course. But I have the distinct impression that you're having a deja vu moment with a traumatically bad guitarist in your own past (or present, gods help you).
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I just scanned the whole thread and couldn't find the "labeling Jamerson a hack", just a dumb line from the keyboard player, which in context sounds like they were being bandleaderly and trying to cool down the argument between the OP and the vocalist/guitarist by making them both "right". But perhaps more importantly, I didn't notice anyone claiming the OP is the issue here. Like I said, rock'n'roll is a team sport, and we adjust to our teammates strengths and weaknesses when we take the field. The OP has a problem. It does not seem to be his fault. But telling the vocalist/guitarist that they are incompetent and 100% to blame is unlikely to improve the situation, and as the OP has said, on the whole it's a band he enjoys playing and gigging with.