Despite quoting it, you skipped the part explaining, why those videos show no valid prove of rockets not working in a vacuum.
Fine with me, I'll accept it as tacit agreement.
The delay you see when rockets launch on TV (i.e., the government or private industry launches) is due to the fact that the plume is not yet contained enough by the surrounding pressure in order for the rocket to take flight.
How about this explanation:
A rocket engine (like many engines) needs a certain time to get up to full working power.
While the thrust provides less acceleration than gravitation (or UA if one leans that way) working the other way, the rocket remains stationary.
Then acceleration will be very slow, but increasing until the rocket engine provides constant acceleration (depending on the rocket engine, that might be adjustable, but for take-off it's reasonable to assume constant full thrust).
The observable result would be a delay until somethings happens, then slow and finally constant acceleration.
On the other hand:
How can a gaseous atmosphere "contain" a plume?
The atmosphere will do some "shaping", as it is being displaced by exhaust, but that effect quickly evens out, as the amount of exhaust is rather insignificant in comparison to the amount of atmosphere surrounding it.
The exhaust is expelled at speed and will be slowed done by the resistance of the surrounding atmosphere. At the same time it will dissipate, as exhaust and atmosphere mix.
Impressive to look at, but irrelevant for propulsion. Pushing yourself off "thin air" doesn't work much better than pushing yourself off a vacuum.
Rockets work in any environment, because they do neither.
This does not apply to model rockets of course, because the weight is not sufficient enough to require it.
Why are you making a special case for model rockets?
The described isentropic flow through the nozzle applies to sound and has nothing to do with propulsion.
Why would isentropic flow through the nozzle have anything to do with sound? It is about thermodynamics not acoustics. The speed of sound is relevant, but not sound as such.
In contrast isentropic flow is relevant to work being done, so it is relevant to propulsion.
iC