Honestly rather than the old party falling apart because of losing MC it feels like they fell apart because of the new guy being a dumbass with a hammer. Leader boy giving the new guy a new weapon at the dungeon entrance was stupid as well. Plus the mage wasn't helping by not picking up the slack from the jack of all trades they kicked out but most of this came down the new guy just completely throwing the fight not them missing the MC
Ehh, I'm not as hard on the new guy.
With regards to the hero's first complaint, you really shouldn't be right next to the guy with the big weapon.
And the second part, where he sent an enemy flying in the hero's direction, I'm also willing to let that slide because he tried to fight with a dagger til he nearly got overwhelmed, doesn't leave much wiggle room to worry about much more than pushing them off you.
Also, that means they got separated quite badly if the hammer guy sends a monster flying forward and it lands near the hero.
Don't even know where the mage and healer are in relation to them but I bet there's enemies between the hero/hammer guy & them.
Altogether, they just seem bad, like, these problems should've been glaringly obvious before MC got kicked kinda bad.