Astrobites is a site where graduate students summarize recent astronomy and astrophysics research papers.
Commenters recommend it as a way to follow current astronomy research, written by people in the field, though it can feel like the deep end.
> If you (the neutron star) gain mass near the gravitational field, it pulls you in closer.
This conflicts with the Strong Equivalence Principle (SEP) in its universality of free fall form, or more colloquially a rule that the trajectory of a self-gravitating object depends only on its initial position and velocity and not on its internal composition (including the evolution of its mass).
There is a good test of the SEP in a system involving the millisecond pulsar https://en.wikipedia.org/wiki/PSR_J0337%2B1715 which is in a hard binary with a white dwarf; a second white dwarf orbits the inner binary. Here is a trace of the orbits https://www.youtube.com/watch?v=oDgfqq_W_uM -- I recommend reading the video description.
Reference 2 (Archibald et al.) at the wikipedia page is an excellent paper.
There are a couple of good astrobites aimed at (advanced, or possibly final-year) astrophysics undergrads at https://astrobites.org/2014/01/14/timing-is-everything-a-pulsar-in-orbit/ (2014) and https://astrobites.org/2019/03/25/testing-einsteins-equivalence-principle-by-timing-a-pulsar-in-a-stellar-triple-system/ (2019)
You're right that the flow of mass in a complex system might be relevant to the merging binary, but in general a stellar black hole's circumstellar accretion structure isn't going to do much to a close-in neutron star except create a lot of (relatively small) nuclear explosions on the latter's surface. There just isn't going to enough material in the accretion…
I'm going to skip the more philosophical part of your question. Or in other words, if you're asking, "is there any observable evidence of something outside the observable universe?" My answer is: no comment at this time.
If you're asking, "what did John C. Mather mean in talking about the edge of the universe?" I'm not John C. Mather, but here's my guess --
The curvature of the universe [1] has been computed from distortion of the Cosmic Micrwave Background and independently using other measurements.
The current consensus is that the universe is indistinguishable from a flat universe. [2]
A curved universe would have an edge, meaning there's a limit to how much could be observed. If it's x light-years until you wrap around the universe, then plant an observer into this universe at a time greater than x years and that observer (if lucky) could observe the same things in two directions by seeing "all the way around the universe."
A curved universe might lead to a "big crunch" since the curvature of space-time is caused by the quantity of matter, or not. Depends on if the matter is tightly packed enough.
A flat universe has no limit -- no matter how far in the future you plant an observer, the speed of light is not fast enough for them to observe the entire observable universe.
There's a relationship between the curvature of the universe, how dense the universe is (how densely packed the matter is), and how fast the universe is expanding (the Hubble Constant). It's…
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> because of dark matter's lack of electromagnetic interaction
This is still an assumption, because we don't know for sure that dark matter is only weakly interacting, but this is what was observed in the Bullet Cluster collision: https://en.wikipedia.org/wiki/Bullet_Cluster
There's a good explanation here: https://astrobites.org/2016/11/04/the-bullet-cluster-a-smoking-gun-for-dark-matter/
The lensing was found to be strongest near the visible matter (stars), and the amount of observed lensing couldn't be accounted for with the total stellar mass alone. So the assumption is that the dark matter, like the stars, wasn't slowed much by the collision. The gas, on the other hand, interacts electromagnetically and slows down in the middle.
That's not to say that the stuff that stars are made up of isn't affected by electromagnetism, but it's a short-range effect and even in a collision, interstellar separation is large (i.e. contrary to what you might think, a galaxy collision doesn't involve all the stars smashing into each other, most of them will just sail right on through). Gravity will dominate in that case.
With adaptive optics it's possible to achieve resolutions down to ~30 milliarcseconds. At 1400 ly that would subtend a distance of 13 AU, so you wouldn't be able to find Earth-like planets. But even if it's theoretically possible to resolve 13 AU separations at 1400 ly, in practice the light from the star is so much brighter than the planet that it bleeds all over the image and completely washes out the reflected light of the star.
To do direct imaging you really need to look at systems near the Earth, maybe a few tens of ly away. Then you can separate the planet from the star by enough that you can hope to mask the light from the star and still be able to detect the planet.
To get some idea of the techniques required to do this, take a look here: http://astrobites.org/2013/06/09/lowest-mass-exoplanet-discovered-via-direct-imaging/
Not OP, but I can think of two reasons – too many LEO satellites can actually hinder views of the night sky[0] and additionally, the risk of kessler syndrome[1].
I'm in no way equipped to give a scientific risk assessment of either, so I'll defer to others to share if they feel it's major concern. But whether these are present issues or not, they're worth considering and thinking about as we move forward with more LEO deployments. The latter issue especially, given that once it occurs, it'll be extremely difficult (nigh impossible?) to rectify in short order.
[0] https://astrobites.org/2020/09/25/leosats/
[1] https://en.wikipedia.org/wiki/Kessler_syndrome