I can only comment on the thread posts.
However, the event appears to be trivial so far as an Mmo exceedance is concerned.
Suggest you check your AFM for any oddball restrictions but, in general, Vmo/Mmo
(a) ought not to be INTENTIONALLY exceeded by the pilot, other than for
(b) authorised test OR
(c) authorised training.
A minor exceedance is a non-event and quite common during descent unless you are maintaining a moderate margin below the barberpole.
From the thead, I would suggest that one probably ought to have done something to control the VNAV excursion earlier and more positively but that's about it.
As a point of note, I think I only ever did one descent in VNAV (-300), wasn't impressed, and always flew the descent thereafter either in level change or, more commonly, handflew. Sounds like Boeing hasn't improved the toy at all ...
Refer to
FAR 25.1505 for information.