Regarding the PLC, you always need their buy-in. Most things in life are really a negotiation. In the examples I put forward, the SM might be seeing something getting in the way of boys achieving the pinnacle scouting experience, so he initiates the change.
It's entirely possible that a boy sees the need for change. That happened with our troop last year. TG started bailing out the SPL, and I started giving him advice on being a better SPL. He pointed out that that wasn't his position. I made it clear that I advise according who I see doing the work, not who's wearing the patch. The SM moved up the election schedule, and no boys disagreed.
I don't view this issue as the boys buying INTO the system, it's the adults cashing OUT of the system. The boys select their PL and if they don't like it, it's not my fault in the first place, nor is it my responsibility to fix it either. I don't set terms, qualifications or anything else in the patrol. They have a problem with their leadership, they have the power AND AUTHORITY to do what it takes to fix it.
The only time I ever had a "full-time" SPL the PL's selected who THEY wanted to support their PL work. They chose one of their APL's to do it. Kinda surprised me, but it worked out well. PL's all retained their positions in their patrols and the one patrol had to come up with a new APL. Very little disruption or commotion i the process. The SPL had to shift gears from being the right-hand man to a PL to being the left-hand man to 4 PL's.
Again, I as SM wasn't involved in the process, I didn't make any adult rules, I didn't give any indication who needed POR (as if that's a big qualification game changer for anyone) and if anything had gone awry, the PL's couldn't come after me for making dumb rules.
Eventually everyone finds a niche that they like and things run smoothly and I don't have to hear any whining or complaining. "Whine and complain all you want boys, I didn't make the problem, and I don't have to fix it for you. Figure it out yourself. Excuse me I need more coffee."
Yes, in the system I use, there's a bit of politickin' going on just like anything else. The TG that gets elected to the NSP as PL after his TG time is up; the APL of one patrol that gets lured away to be a PL of another patrol; etc. With no rules applied, the first time a patrol "shanghaied" a patrol member of a different patrol to be their PL, there was a big to-do. It was the NSP that elected the older brother of one of the new boys to be their PL. It worked out fine.