I think Petra meant that McGee didn't lead an episode/the team without the others being there, but not sure. Either way I think he did both what you're describing & leading the team without the others. We know at least Tony & Bishop have done things solo. Nick was on his own for a bit too but leading a team is a whole lot different to what he was doing working undercover. They've all had an episode where 'your case, your lead' came into play from what I remember. Nick may not have...

McGee has grown a lot & I think he could grow as a leader for a team. But in enough time to lead the show? Then for the actor/character to lead the show? I don't know... but I haven't seen the episodes & no idea what their plans are so maybe this other character is who they're planning on bringing in to replace Gibbs. It makes sense. It isn't like they explained why they were bringing the character in.