Over the years there has been much speculation and confusion with appropriate use of FLCH. When I have come across such confusion (many times in 12,000 x 777) I have explained that it is an IAS lock, much the same as the IAS button on the MD 80 series. There are a few other things that attach to it but essentially it will try to maintain selected speed.
I agree with your explanation and I think the important point that some may not realise is that it’s the
elevators that are actively controlling the “IAS lock” and they will be trying to “maintain selected speed”. The Autothrottle may be “asleep” during a FLCH Descent, even if it is armed. Ask Asiana.
As I said earlier, FLCH mode is one of the few where the Autothrottle does not control speed.
In fully auto-coupled flight, everything should work out but if you decide to fly manually and then deviate from the flight-director.......watch out!