Ethan Siegel (Starts With a Bang) gives a good explanation in his article about how some galaxies don't have supermassive black holes. (https://www.forbes.com/sites/startswithabang/2021/04/23/ask-ethan-why-doesnt-every-galaxy-have-a-supermassive-black-hole/?sh=55e747d23374)

>It’s an eminently reasonable thought that every galaxy in the Universe should have a supermassive black hole, especially considering that the processes that we think lead to their formation:

• early, very massive stars form,
• some go supernova and some directly collapse,
• their remnants dynamically interact with the surrounding matter,
• causing them to sink to the proto-galaxy’s center,
• where they merge,
• and then these “seeds” of supermassive black holes accrete matter and grow,
• leading to what we observe today,

>ought to occur everywhere a galaxy is present.

So the answer is "sort of". While the super massive black hole isn't what holds the galaxy together, it may be that it was the seed that pulled the initial cloud of gas together to start the galaxy.

Time dilation due to gravity wells has been well tested by satellites in Earth's orbit. They aren't as deep in Earth's gravity well and experience less time dilation. It's very tiny, but measurable.

Even more, specific, the orbit of Mercury is affected by time dilation due to how close to the Sun it is in a very noticeable way. This was a real mystery - people speculated there had to be another planet pulling on it, named Vulcan - until Einstein proposed Relativity as the correct answer.