Yes -- banishment says it is "a more powerful version of dismissal", and it doesn't override the 20% chance, so that is in fact how it works.
Given that your question is tagged "pathfinder", by the Rules As Written there is no way that the PCs could wind up in Maztica or Dark Sun or Greyhawk (as these are all D&D settings).
A very literal-minded DM might argue that, since no destination within the Material Plane is specified, the PCs arrive at an entirely randomly chosen location, which is overwhelmingly likely to mean they show up in airless deep space, far from any sun. Let's hope they have a silent greater teleport spell ready!
A less literal-minded DM might treat the dismissal effect as though the PCs had cast plane shift -- they'd appear "5 to 500 miles (5d%)" from the place they left the Material Plane.
The Pathfinder Wiki has some words about the Pathfinder settings that officially exist on the Prime Material Plane. If you're determined that the player characters arrive on the surface of a human-livable planet, but you'd prefer that it be a randomly selected planet, your choices include Golarion, Earth, Androffa, Kasath, and, er, Carcosa.
Of course, this is your cosmology; if you'd like to rule that multiple D&D settings exist in your version of Pathfinder's Material Plane, that is your prerogative.
There is no reason to assume that the Spellplague impacted other realities.
For several reasons
Crystal Spheres aren't part of The Realms campaign setting, they are added by Spelljammer.
First and foremost, understand that the concept of Crystal Spheres, and every other setting existing in a different one floating out in The Phlogiston is NOT core material to The Realms. It was added for the Spelljammer Campaign Setting as an optional way to tie all of the different settings together. In the FR wiki article linked in the question, note that the only source cited is the Concordance of Arcane Space...which is a Spelljammer Rulebook. The Planescape Campaign Setting offers an entirely different optional way to tie the settings together. This was explicitly done to let you play in multiple different settings with the same characters. It is, essentially, a hack applied on top of the Core settings.
The core settings (Forgotten Realms, Dragonlance, Greyhawk, Eberron, etc) all operate under the assumption that other settings either don't exist, or are sufficiently disconnected as to not matter. You never see a setting book explaining how this thing that happened in Eberron impacted these other things that happened in Faerun. They simply assume that the setting you are playing in is the only setting that matters.
Thus, outside of these two optional settings, travel between the two is impossible and Oerth and Faerun may not even exist relative to one another. (In a game played in Faerun, Oerth and 'Greyspace' may not exist at all). Based on that alone, each individual setting can be considered to be entirely cut off from one another, and cannot impact each other. Because without one of two optional campaign settings, no connection between them exists.
Nature of the Spellplague
The Spellplague was the result of a breakdown in the Weave of Magic caused by the murder of Mystra, goddess of magic. With no deity to manage the weave, it went rampant and caused a huge mess.
Thing is...
In Greyhawk, Boccob is the god of magic. In Krynn, it's Lunitari, Nuitari, and Solinari. In Eberron, there is no god of magic, magic is (theoretically) derived from Siberys, the golden ring around the planet (and dead progenitor dragon).
And, according to the Spelljammer rulebooks (concordance of Arcane Space p17-18) deities have no impact whatsoever on any crystal sphere they do not hold sway in. Unless they have a pool of worshipers there, or a powerful cleric who can make a connection, they can't interact with that sphere at all. The death of a god recognized in only one Sphere cannot impact another sphere.
Differing cosmologies
The settings in D&D are extremely different. In terms of settings that go into significant detail on cosmology...The 'World Tree' cosmology was something unique to the Faerun setting.
Greyhawk was left intentionally vague in terms of cosmology. Krynn Cosmology looks like this. Eberron Cosmology looks like this. Old Faerun Cosmology (World Tree) looked like this, and the new version like this. See how different they are? There's no way these are the same planes in the same Astral Sea.
In Conclusion
The simplest answer is that, according to the core settings, nothing that happens in one setting impacts anything that happens in another setting, because they are different settings. The fact that two additional settings (Spelljammer and Planescape) were invented to let you optionally glue them all together doesn't change the fact that they are entirely discrete settings with their own history and events that do not impact each other.
Best Answer
Yes, and also No, but sorta mostly Yes
The default setting of D&D 5e is “The Multiverse”, and as far as I can tell this contains all possible worlds, including words with different cosmologies. This is a result of The Multiverse being the default setting, and the canonical notes in the DMG that the planes can be chosen à la carte by the DM when designing a campaign setting.
The latter means that each setting can have a different planar cosmology, as designed by the DM. The former means that even such worlds share the same Multiverse, despite differing planar cosmologies.
The conclusion then is that no, the existing published worlds don’t have a shared cosmology… and also that there is another version of each that does.
Aside, another consequence of the Multiverse is that one group’s Forgotten Realms (to grab one example world) is a distinct part of the Multiverse from each other group’s Forgotten Realms, all of them part of the Multiverse. Basically, there are infinite variations of each world, with infinite variations of cosmologies, and infinite permutations of sets of these with shared and cosmologies and without.
The upshot: practically, players consider settings and cosmology to be somewhat shared
In practice, many groups appear to see the Forgotten Realms as one place by default, and seem to assume that there is one shared cosmology. Most people don’t seem to even need it to be clearly defined in the first place.
Consequently, you can write and publish with the confidence that most of your potential audience will be happy to accept the assumption that (e.g.) the Forgotten Realms and Greyhawk share the same (e.g.) Bytopia, if your product makes that assumption.
Put another way: the rest of them — that is, anyone who has opted to take the DMG’s advice and craft a separate cosmology —may still find use in cherry-picking your material to suite their needs just as they did with the DMG chapter on cosmology design.
You can even encourage this, by giving your default assumptions but taking the DMG’s lead and noting that everything presented is optional and can be mixed-and-matched. Even if your assumptions make that hard to do, you should be fine — anyone doing an à la carte treatment is already confident adjusting your material for you.