While there was a lot of explaining and, again, I thank you for providing it, do you have a video that explains the video that you posted?
lol
Cutting to the chase, you're saying two things - I think, one a black hole can never get big enough to collapse any further as do stars, correct - and - two, there is never any way for a black hole to consume it's host galaxy, double correct?
Let me see if I can try to explain it a bit differently and start with some fundamentals.... and I'll tell ya, a lot of this contradicts what I was even taught in high school long ago.
To understand what a black hole is, you have to understand some quantum physics.. but nothing too in depth, just the shallow basics. The very little really does control the very big...
Grade school science books always depicted and showed models of atoms in very poor scale.. They'd show something like a group of tennis balls at the center nucleus (representing protons and neutrons) and the orbiting electron would be something like a sweet pea.. and be located somewhat close to the nucleus like two or three tennis ball diameters away at that scale.
In reality, if you were to scale up an atom so that the protons and neutrons at the center (the nucleus) were the size of tennis balls, the electron would be smaller than a particle of smoke and its orbit would be about 3 to 5 miles away from the tennis ball. (YIKES!).. Yes, that's not a typo.. three to five MILES from the center at that scale. Everything between the electron and the nucleus (center of the atom) would be empty space.. Not space filled with air because air is just matter made of atoms, but actual empty space.. a complete and total void of all matter.. a vacuum. So in reality, an atom is 99.9999999999999% empty space. (maybe I should have added more 9's?)
An electron orbits the nucleus at about 2000 kilometers
per second in actual real speed.. (about 1% speed of light) so at the small scale of atoms, for all practical purposes, the electron is going around the nucleus so fast (2000 km/s) that its just a spherical blur like a fan blade in your living room.. only many times faster. Its like an orbital halo called an "electron shell".. its so fast, it might as well be solid sort-to-speak.
Due to something called the "Pauli Exclusion Principle", the "halo's" of individual atoms can not overlap each other (think the Mastercard symbol with the overlapping circles) Like a bunch of ping pong balls in a container.. they can't go inside each other's shell.
But in simplistic terms, if you squeezed a bunch of atoms together with enough pressure (think core of large star), you could compress the electron shell into such a small space that the electrons would literally be pushed into the protons and combine with the protons to form more neutrons. (1 Proton + 1 Electron = 1 Neutron for all basic purposes)
The atoms fight this squeezing pressure and we call it the "Electron Degeneracy Pressure".. Like squeezing an egg in your hand, the egg puts up a fight, but if you squeeze hard enough, it lets go, the shell breaks, and it collapses.
When a star like our Sun collapses, it doesn't have enough matter at its core to overcome electron degeneracy pressure so it leaves behind something called a White Dwarf.. they're normally about the physical size of the Earth but there's so much matter that it its like having thousands of Earths all squeezed into the same physically sized ball. Electron degeneracy pressure is what keeps that ball from collapsing any further.. A White Dwarf is a very dense object, a teaspoon of White Dwarf matter would weigh about as much as an 18 wheel semi-truck.
If the original star contains about 10 times the mass of our sun, it will go supernova and the explosion will compress the core.. and while it happens very fast, the core will first become a white dwarf, then the Electron degeneracy pressure is overcome and the electrons are squeezed into the protons to form neutrons.. and a Neutron Star is formed.
Remember all that empty space an atom has inside it? Well now all that empty space is gone.. because all that empty space is gone, it is now occupied by other matter. Neutrons are positioned right next to other neutrons with no extra space between them.. Its one big clump of neutron matter and 1 teaspoon would weigh as much as an entire mountain!
The rough equivalent of 10 White Dwarfs (ball the size of earth with thousands of times more mass), has now been compressed into a ball just around 10 to 15
miles in diameter.
The thing that keeps neutrons from being squeezed together is something called Neutron Degeneracy pressure.. So we understand that an atom looks like a round spherical ball because the electron is orbiting the nucleus so fast it makes it seem like a solid surface.
Neutrons are the same way only instead of protons and electrons, neutrons contain particles called Quarks. Each neutron has three quarks inside.
This following is where known science meets hypothesis.. We're not entirely sure what happens when neutrons are squeezed with so much pressure that they also combine. What we do know is that the resulting mass has enough gravitational pull to reach its Schwarzschild radius and no light (electromagnetic energy) can escape for us to get information from.
The most accepted idea is that once Neutron Degeneracy Pressure has been overcome, it all collapses to an infinite point.. But this has never made sense to me and a lot of very important physicists are now starting to believe in something called a "Quark Star".. But in practical reality, there would be no fundamental difference (to us anyhow) between a Quark Star and or whatever comes last. IE: The absolute maximum density if it even exists.. To us, once you pass neutron star, no more information can be gathered because no light or other information escapes.. (we'll skip the whole Hawking Radiation thing for now).
Can a black hole swallow its host galaxy? Sure it can, and it will eventually.. in time frames measured in Trillions of years.. (universe is only around 13.7 Billion years old so don't cancel any appointments)
But remember, a galaxy with a mass of 100 billion sun-like stars will have the same gravitational effect on other galaxies as a single black hole with a mass of 100 billion sun-like stars. (But would probably be really cool to see the gravitational lensing of such an object)
Extrapolating even further, maybe the whole Quark Star thing is correct and they hypothesize that even quarks are made up of even more fundamental particles call preons.. and maybe those are made up of something..
This whole thing has always reminded me of those Russian Nesting Dolls where you open one up, and another one is inside.. open that one up, and an even smaller on inside.
And maybe, each stage of collapse (Electron Degeneracy > Neutron Degeneracy > Quark Degeneracy > Whatever) leads to an end and maybe it takes all the matter in the universe to reach those kinds of pressures, and when they are reached, they rebound and explode again into another Big Bang. Matter is created, galaxies coalesce, life forms, and eventually intelligent beings discuss it all on some machine invented by more intelligent beings.