The GAs answer should be that you are more than welcome to save her a middle seat next to you, behind the exit rows. :)

Technically the GA is correct. Boarding position isn't always just about the seat. You are guaranteed a seat whether you board first or last. You may or may not have somewhere to stow your carry on depending when you board.

The first lady saved overhead bin for the second lady...so, effectively second lady boarded early.

I completely disagree with the bold statement. I've been on flights that were overbooked, and the GA allowed everyone to board, leaving a person or two wandering the aisle with no seats available. They were asked to leave the flight. Although rare, I wouldn't always count on a seat if boarding last. And to answer the obvious question, no there did not seem to be any non-revs on the flight.

Did they ask for volunteers for VDB, or just IDB some people?

From the description, I would assume that WN handled it like musical chairs, with whoever didn't have a seat getting the IDB. I guess the lesson to be learned here is that if you're boarding very late, don't pass up an empty seat no matter how undesirable in the hope of finding a better seat farther back.

This is basically what happened. The person left standing was IDB.

Unless you want more than the standard IDB compensation. I know if they IDB'd me without asking for volunteers first, I'd be explaining to them how I know they just violated the law and owed me significant compensation.

I'm going to guess that it wasn't so much she saved overhead space as the space she was guarding wasn't needed. If it was needed before her friend boarded I would be willing to bet that her attempt would have been thwarted. Either way, she played within the rules and apparently no one challenged her. Maybe bin saving should be spun off to it's own thread. :D

Fair enough but to completely disagree suggests that it's a crap shoot when you board the plane last. Thousands of successful enplanements every day suggest that it is far from a gamble.

Flight was maybe nearly full, prob a handful of middle seats open at the rear of the plane. Maybe 3 or 4 passengers attempted to put their bags in the bin but were confronted with first lady (I'm saving the seat and the bin space, there's plenty of room in the back). FA never got involved; just observed.

We have one already that's close enough: Overhead bin rudeness check