I'm kinda chuckling over the names. Road Captain/Ride Captain, Tailgunner and now Corner Marshall.
I'm low maintenance when it comes to riding and I see everything I want to see all by myself or sometimes with one to three riding buds. Never any more than a herd of four. We never feel the need to entertain ourselves because we mostly say what we think in that is what we believe or we don't say anything and it's all good. Large groups of riders are tiresome in that after a while you get tired of entertaining them. U-turns will resolve that I've found. I still support some of the causes that are important to me like others that posted above. Either online or show up give them cash, get a tee shirt, look over the bikes and scoot on out of there.
Sorry, I can't shed light on the Corner Marshall (???) and their function as posted by the OP. I ride to rid myself of governance. Not to add to it.
You're right, it is kinda funny. Generally, one of the more experienced riders will step up and volunteer to lead. If it's a less experienced rider volunteers, the group will generally suggest hanging in the middle for a bit. Another experienced rider will choose the rear. The conversation generally follows; "Okay, I'll lead if everyone's good with it ... okay, I got the rear then. John, you wanna hang with the leader then?" It's funny, but every group finds a hierarchy, undeclared leaders and undeclared followers. Those that join for a ride that don't like the current order are welcome to find another group. When I join a group for a ride, they don't know me and I may only know one or two individuals, I just let them set the order and pace. It's called getting along.
Sorry, I wandered there. We never use those terms, although some groups do. "Lead", "Rear", and set a loose order for the middle, and let's ride! We do make a point of talking about things at stops though, like if there are any safety concerns, pace issues, or whatever.