The standard boss-worker relationship of today could in theory arise in a free society (though it would likely fail in the labor market with appropriate competition). As for arise from, it would seem unlikely except possibly from individuals with certain mental conditions.
So your argument is that a boss/employee relationship is less efficient than a worker's collective?
I would argue that you'll never steer a ship with a dozen rudders.
No, it could just be that less efficient-or-no, workers with a free and easy choice to work somewhere that they have a say in the governance and the decisions that affect their lives will find that option so much more desirable that you will not be able to convince any to work for a boss. It likely wouldn't be a binary, rather you could say the equilibrium point would make small scale co-ops and other arrangements much more likely than it is today.
This. The boss/employee model will have to face competition with other models in the labor market.
As for collectives, there is nothing stopping from collectives from having leadership; its just that the nature of the leadership would be different.