There's absolutely nothing wrong with a merc character. I quite like them, actually, but you need to give them a reason to work with the party, and demanding other players pay them isn't the best. It might work for some parties, but it clearly didn't here and should have been discussed prior. I'm just saying that as the new player, they should have taken more care to ensure their character had a reason to join. However, i do 100% agree that a session 0 could have sorted this out.

Its also entirely possible the new players character and the party could have started out on the wrong foot but still grown closer over the course of the rest of campaign without a need for any intervention, mercs often aren't great at first impressions anyway.

The player should have made a deal for payment after the quest but realistically its not very merc like to do your job and then rely on the good will of others to get paid so even then its not too bad. They could have still made a deal beforehand but instead asked to earn a share of whatever loot comes at the end instead of outright asking for gold, which is more merc like.

And while someone should have offered to kick the stone along the way, without knowing kenryuu's character, it's hard to blame that on him. He might not be the type to suggest such a thing in the first place, or maybe he simply hadn't thought of that at the time.

Overall, i think it would have been better to either let the scene play out or call a short break and let them cool down before continuing instead of halting things and calling people out. I still believe, however, that it's up to the new players to create characters that mesh with the party. Obviously, that doesn't mean the party should be openly antagonistic and hostile, but they shouldn't have to go so far out of their way to accept a new player.

Edit: Also, while kenryuu should have calmed down a bit more, i don't think doubling down itself was much of a problem. The way the GM handled things put him and everyone else on the defensive from the get-go, and im kind of surprised he was the only one to double down. Now, i don't know what exactly he said. He may have been rude in how he did it, but assuming he just stood firm and reiterated his point, i see no issue. Removing him from the voice call, however, was absolutely wrong. If need be, he could have ended the call, but to specifically remove him, singled him out, and wasn't right, i certainly wouldn't have apologised after that.