Just because he isn't the leader doesn't mean the Lightforged wouldn't respect his advice. And no shit Velen screwed up. That was his whole arc witb Illidan on Argus. He's MIA in BfA because he is quite powerful and would shit on forsaken troops. Again shoddy writing but that is par for the course.
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And they didn't seem to bothered when Illidan smoked their holy Xe'ra. If they were as zealous as you say they are, they would have cared a bit more don't you think?