In a vacuum solution, there is no "observable universe's light", there is only the central mass M at point p, and some test observer who can probe point != p.

If there is starlight falling onto the black hole we adapt the metric e.g. Schwarzschild -> Vaiyda (incoming) [1], which models this incoming light as spherically symmetrical, ignoring Olber's paradox, and equipped with no wavelength, charge, or rest-mass: a "null dust". The major practical difference is on the wavelength stretching/squashing that light would experience, but the "null dust" does end up with lighter or heavier "raindrops". In this approach, and more realistic but still very theoretical ones, there are families of accelerated observers, including ones orbiting at ISCO, that will be trying to move through a torrent of heavy-raindrop incoming null dust.

"Freefalling into a large black hole should not be inherently dangerous". No, a massive radial freefaller in the Vaiyda model will get splatted on the back windscreen by heavy raindrops. More physically, a subluminal radial infaller will get sunburned by distant starlight trying to race past it, and additionally any other faster-moving infalling mass, such as cosmic ray protons.

"It's very hard to freefall into a black hole". One can do brief course corrections as one approaches the black hole, and still be in freefall when one shuts off the short-impulse thrusters. The adaptation of the resulting worldline isn't especially hard, and one can simply take its cutoff at the final course-correction. This is easy enough to see with a Minkowski diagram -- the accelerated parts of the worldline will be curved, the freely falling parts will be straight-lined: this image is frequently encountered in resolutions to the Twin Paradox where physically plausible acceleration is introduced.

> evaporation is basically nonexistent for any large black hole

In a theoretical model, like Schwarzschild, we literally have all eternity to trace the black hole's evolution, of course. The central finding of Hawking's 1974 paper is that arbitrarily large Schwarzschild black holes with vacuum (or classical electrovacuum) substituted with a noninteracting scalar quantum field will evaporate in finite (if long) time. Follow-on work has generalized to different quantum fields and black holes that form by collapse or which have non-negligible spin or charge parameters.

More astrophysically, we do expect to find Hawking greybody radiation around black holes of all sizes in our universe. The greybody temperature will be cold, especially for more massive black holes. Crucially this temperature is much less than that of the cosmic microwave background (much less typical interstellar or intergalactic-but-in-cluster media), so the Hawking radiation cannot cause these black holes to shrink. The origin of the Hawking quanta extends well outside the event horizon, so a well-planned hyperbolic orbit with well-shielded and sensitive sensors should pick them out. There are plausible natural phenomena which that idea roughly models, or conversely, it could be revealed by the inverse compton scattering spectrum of a very large weakly-feeding black hole (Sgr A* is not hopelessly far from that!).

I'm sorry, I just can't understand what you are trying to say in your second paragraph's second sentence. (The first sentence is just observing that collision with the singularity is in the future of every object crossing the horizon. "Time axis" depends on choice of system of coordinates, and one has total freedom there (including using no coordinates at all), as in any General Relativity problem. I don't see how to relate that to other infallers though. "Everyone gets squashed into the singularity" is what you are trying to say? So? It's not like one can have a conversation when one is part of a singularity.)

> How does one determine they are in the universe-sized black hole?

One looks at the sky and sees everything in it apparently contracted to an extremely bright high-energy point. If one sees a spread of galaxies occupying different solid angles on practically the entire sky, with smaller angles relating to higher redshift, one is not in a black hole, one is in an expanding universe.

In a vacuum setting, where there are no galaxies at all, one would have to measure the local spacetime curvature as discussed in https://physics.stackexchange.com/questions/109731/how-to-me... -- particularly the overview of the point raised by JL Synge in his textbook 1960 Relativity: The General Theory; Pub: North-Holland that one finds in one of the upvoted answers. As noted in several of the answers, with some care one can measure an angle deficit. Alternatively one could track the evolution of a freely-falling spherical dust cloud's oblateness/prolateness: https://math.ucr.edu/home/baez/gr/ricci.weyl.html -- in a vacuum expanding spacetime sphericity would be maintained, whereas in a vacuum black hole the cloud would become ellipsoidal. Even in a universe-sized black hole, starting far from the singularity, the cloud would on human timescales develop a "nose" pointing in the direction of the singularity.

> What difference would it make?

The interior of a universe-sized black hole is not compatible with life (or star formation).

(Life (and stars and galaxies and so forth) could in principle exist outside a universe-sized black hole, but would notice that the views towards the black hole and away from it differ remarkably).

Why does this matter? It addresses your point that a universe "close to big crunch, a lot of very heavy black holes begin to merge" is an objection to stars and galaxies popping into view after than had receded behind a cosmological horizon, which also sidestepped the point that as a black hole horizon recedes, stars and spaceprobes and so forth do not pop out.

Black holes and cosmological horizons are just different.

See the section, especially the second and third paragraph at https://en.wikipedia.org/wiki/De_Sitter%E2%80%93Schwarzschil...

We got to this difference by you rejecting isk517's perfectly reasonable comment, "Things moving super far away are lost but should still be out there, things falling into a black hole disappear and then once the black hole evaporates are gone forever", which I have just been expanding upon. In particular your objection was "In both cases it is due to curvature of space, so I think these are essentially the same case". Above is how they are not the same case, stemming from the curvatures of black holes and expanding universes having different sign. Finally, you asked to "hear something verifiable on why there's a difference and why only one of these is susceptible to information paradox". Which I was trying to do. The tl;dr is that the contents of a black hole drives Hawking radiation even if the radiation's associated greybody temperature is too cold for evaporation; the expanding universe's contents cools and when the associated ~blackbody temperatures drop below that of the Hawking radiation, black holes will fully evaporate. That evaporating black holes have a greybody spectrum unrelated to the microscopic details of what fell in is the information loss problem in a nutshell.

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[1] https://en.wikipedia.org/wiki/Vaidya_metric#Ingoing_Vaidya_w... (eqn 15 with r ~ 6M).