Planar Structure and Physics

If I am in a weightless plane, do I need to hold the black holes I am storing there together with glue?
Nope. Gravity can exist within a weightless plane, it is simply not inherent to the matter of the plane itself. Even if that were not the case, the result of the forces that created the black hole in the first place would remain a mass sufficiently dense and compacted as to hold itself together in the absence of an external, separating force.

What does a plane look like from the outside? What happens when you smash two of them together?
A demiplane usually looks like a sphere from the outside, with specific visual characteristics based on the nature of that plane. Infinite planes have no "outside" that can be seen from any practical position.

Planes that collide tend to "emulsify" briefly - that is, there's a crossover in the collision zone, with elements from each appearing. Actual collision doesn't happen, but some planes naturally brush against one another - the Inner Planes all brush against a few, and there are places in the Outer Planes that show a bit of bordering as well.

Subjective gravity states "each individual". What happens when you get two minds in one body (via schism or possession or similar), and they want to go different directions?
The stronger of the two minds prevails. When both minds are equally strong... have you ever heard the one about dropping a cat with a slice of peanut-buttered bread on its back?

Do time traits affect time stop and similar?
If the book doesn't say so, then no they do not. Planes with time traits should naturally interact with time magics in some fashion regardless.

==== Undead and constructs are immune to Fortitude save-effects unless they work on objects too. A strongly positive-dominant plane offers a Fort save to avoid exploding, but specifies creatures. Can these two creature types get infinite HP on the Positive Energy Plane? ==== Positive energy on the Positive Energy Plane should affect objects as well, with the same dramatic results (glowing and exploding). This appears to be a rules oversight, and an unfortunate one. Positive energy should affect undead in the same way that negative energy affects the living, but of course, these books were written from the perspective of having a standard adventuring party, and don't do well at addressing issues of monstrous entities brought there. This is why, for example, ravids have no immunity to their home plane's effects and deal damage with their positive energy lash. I would recommend that DMs treat the Positive Energy Plane as hazardous to both undead and constructs, and not at all hazardous to ravids.

==== Both Planescape (IIRC) and 3.5 describe the Elemental Planes as both infinite and bordering other Elemental (or Para-Elemental, or Quasi-Elemental, or Energy) Planes. Hence, where the Water Elemental Plane borders the Air Elemental Planes, you'd have steam. But how can something be both infinite AND border something else? ====

Well, I'll explain by analogy.

Start at 0, and count upward by ones. The set of numbers you count (if you could count forever) would be "infinite;" that is to say, it is uncountable and unending. Nonetheless, no matter how high you count, I always have one "end" - the zero cannot move. Now start at zero and count downward by ones. Again, you have an "infinite" set of numbers - you will never reach a number so low that it cannot be bested by subtracting one - yet it has a defined limit.

Compare both of these to the entire set of integers - it is also infinite, and yet logically must be larger than either set individually, comprising as it does both of the previous infinities. Now take the real numbers - infinitely more infinite than the integers.

The way of the planes is such that there's no physical (0,0) at which all four elemental planes would originate and move away from, nor is there a point (X, 0) or (0, Y) that is "fixed" in planar space where the border is located. Rather, these realms are infinite, and within these realms exists a border with another, which can be traveled to but is not a defined PHYSICAL limit.

The same applies for the Outer Planes in some measure. There are gates, so it's not exactly a border. But there are also River Styx and Oceanus, the Yggdrasil and other things that link stuff. How can you picture, for example, Oceanus passing through Arcadia and Ysgard? Do the shores form a continuum (and, thus, there is a real border, like in the Inner Planes), or does only a tiny fraction of the river exist in both planes (imagine: patch of land-river-patch of land)? Both are somewhat strange propositions! ==== ==== Again a metaphysical quirk - the shores do not (typically) form a continuum, nor do the rivers, the tree and the mountain have defined points at which there is a changeover. The analogy for this would be to go into Paint and draw a series of lines one next to another that shift from (255,0,0) to (255,255,0). Looking at such a band of color, you could tell me that you started at red and went to yellow, and you could probably point out orange to me, but is where you think orange begins the same place that I think orange begins? Do you think you would pick the same place at a glance, every time? And when you have determined where orange begins, will you be able to say, conclusively, looking one pixel to the left, that the line to the left is definitively "red" and not "orange?"

In Planescape, I remember some locations changing planes because of a shift in alignment. A place in Mechanus that became too Good-aligned could suddenly become part of Elysium. Now, how do that work? Is there a land transfer, or are only buildings and/or features transfered? ==== ==== There can be a land transfer from time to time, too, though the land would gradually take on traits in accordance with its new environment. A transition from Mechanus to Elysium is almost certainly out of the question, however - goodness appearing in pure law would move a location into Arcadia first, then Celestia, then Bytopia. It's not the most sudden of transitions. There won't be a crater left over or a town crushed under SURPRISE TOURISM ROCK, though.

Why are there so many themed planes (Air, Fire, Water, Earth, Negative, Astral, over a dozen different Alignment Planes, etc.) but only one Material Plane?
The Inner Planes are the "building blocks" of matter itself. The Outer Planes are the embodiments and reinforcements of cosmic forces and belief. The Prime Material Plane is the fulcrum of it all, with the Transitive Planes being mediums for matter to join together and belief to provide meaning and purpose. In other words, the Prime is the "sum" of the rest of the cosmology.

That said, there are other Material Planes. They can be traveled to by entering the Deep Ethereal, or trekking into the secret routes in the Plane of Shadow. Eberron is commonly held to be a different Material Plane from that which houses Toril, Krynn and Oerth, for example. The distinction between other spheres (same Prime, different "solar system") and other Primes isn't well-explored and a great many worlds are considered to share the same Material Plane, but others do exist and have been seen.

The Inner Planes are also the original planes. The Prime is the conflux of those forces joining (the 0,0 on the coordinate axis, if you will) and provides the belief that underpins the Outer Planes. Depending on where your interests lie, it's either an uninteresting parasite (Inner), your world (Prime) or your life source and battleground (Outer).

==== I know that Arcane magic changes based on which plane an individiual happens to be on at the time, bud do psoinics change also? If psionics do not change, what do you think is the best method to get the player who started up a Sorcerer to not hate me for making this a Planescape game? ==== Psionics don't change, per se, but they can become more difficult to use on the Planes. Certain powerful entities can exert a sort of mental pressure that increases the PP cost of psionic abilities. Similarly, on the Outer Planes, the greater the difference between a psion's alignment and that of the Outer Plane he or she is on, the more it can cost to activate and/or augment.

Don't be afraid to give some planar creatures psionic competencies. In the past, many outsiders had psionic abilities, but as psionics was not a component of 3.5's core rulebooks, psionic monsters did not appear in Core.