They'd be disadvantaging themselves because they'd be competing as ironmen with a group of x when they actually have a group of x-1 and have to carry a new member to make up for the one they kicked

The person that is the most disadvantaged is the person who got kicked since they would be in a group of x when they are only one person and need x-1 extra members. You are right that kicking still disadvantages the team but I don't think giving players incentives to kick is a good idea which is why I think item tagging is a good idea.

No incentive for the person being kicked to do so. Could lead to black market for account services and account selling but that'd be up to Jagex to deal with since it's against the rules.

What do you mean there is no incentive? They would be able to gather gear/stats nearly instantly with no repercussions and would be directly competing with people that would have to get things on their own. How is that not incentive?

Same as main game, they can ban your related accounts too

How do you distinguish?

Sounds annoying as fuck because it means the PvMers of a group can basically hold the group hostage if drama happens. Like the potions made by any skillers would be used by the PvMers so it doesn't matter if the skillers tag their items because they've already been used to get the drop that's tagged by the PvMer.

This is fair argument but what if you could give ownership to other players providing that they have been in your group for X amount of months.