First of all, the D&D multiverse is not consistent between or within editions or settings or individual writers' output. As such, trying to figure out how things work "in D&D" or even "in Forgotten Realms" is kinda futile. The body of TGD articles taken in entirety and the Tomes are likewise collaborative; also, sometimes they discuss a D&D book and other times they offer revisions.
Thus, without narrowing down your questions to a book or a thread, it makes sense to offer some general considerations/recommendations.
allanlerouge wrote:1) How are Outsiders "created" ? Are they former "Prime" souls that became Petitionners, then some other type of Outsiders later on, is there some "artificial" creation process existing, like God-made, or by some kind of machine ? Something else ?
Outsiders are beings from alignment planes. Once you're dealing with them, you're dealing with alignment, and you should decide how it works in your game; then decide what happens to souls after death.
There's something of a majority opinion that being transformed into a blade of grass warmed by Lathander's light forever is
worse than being transformed into a dretch and having to earn your way up (the way it's sometimes flavored). If Cosmic Good and Cosmic Evil are factions unrelated to actual good and evil, this works, and you can play mortals caught between fiendish dog-eat-dog bureaucracy/murderfest and celestial oblivion. If, however, Evil is evil, being opposed to rapemurder as players and characters normally are shouldn't automatically put them on Team Oblivion.
Aesthetically speaking, I am very much opposed to death being in any way beneficial, or continuity of consciousness for celestials and demons. Death is bad. It may come with a nice severance package, but it shouldn't be a powerup, and past lives isn't something I find appealing at all. My preferred version is that a god expends power to specifically retain you as a petitioner when you die as thanks for service (like in Greek mythology some mortals get to live on Olympus), or if you're individually powerful, you can sorta hang around in pocket reality like an unplayable Markov chain twitter bot, but to actually become a Celestial or a Fiend, you need to do it alive. Other souls meld with the plane, and Celestials and Fiends spawn from planar material to feed ephemeral bodies into the planar war machine. Other Outsiders can live there and breed 'normally', even though they might have human intelligence. You can raise hellkittens (cats are already evil IRL) or achaierai chicks. Baby succubi are a nope.
allanlerouge wrote:2) What is the relationship between Gods and Outsiders ? I get it that Gods prefer to live in the Outer Planes because of the Divinely Morphic trait (and being creatures of Belief and Concept themselves, being in the Conceptual planes seems logical), and in the fluff gods seem to have outsiders serving them. But not every Outsider serve a God, right ? So what make one serve a god, and another not ?
Gods in D&D and in FR in particular come from various backgrounds. Most of them are definitely not "creatures of Belief and Concept" but anthropomorphic beings with mortal interests and priorities. Specifically FR gods are all real-life-evil. Gods don't prefer to live in the Outer Planes because of the Divinely Morphic trait, gods live there because that's where the D&D writers decided they should live (in the supernatural worlds where souls go), and the morphic thing is a tack-on.
3e Gods also need a massive but easy nerf to cut out the DM fiat bullshit. Pissing off gods without getting no-save-killed, killing gods and taking their place should all be possible. It's what happens in the source material, and the game should support that. If "invent a spell that kills Tordek the Dwarf Fighter with no save, then cast it" is something a god can do as a free action, it's obviously not supported.
Gods are Outer Planes landlords and faction leaders. Outsiders join them out of some personal alignment-colored reasons, but they don't have to.
allanlerouge wrote:3) What's the extent of the impact of an Outsider's plane of origin on the subtype(s) of an Outsider ? And on its behavior / values ?
Outsider types should obviously correspond to the plane of origin. The exact measure of influence on behavior/values is up to you to decide; however: D&D is a
roleplaying game about stabbing people in the face and taking their stuff. If you can't stab fiends without feeling guilty about it, that's bad. If you can't talk to fiends, that's also bad.
allanlerouge wrote:4) Why the fuck are the Angels able to come in all the "Good" flavors, but other "sided" outsiders mainly are 1-flavor-only (ok, some are 2) ? What create them ?
Well, the counterpart term to Angels (I prefer Celestials) is "Fiends". Fiends come in all Evil flavors. There are notably fewer flavors of Celestials, because you generally don't interact with them as much, and because there's only one way to do good and that is collaboratively. You want bureaucrats, you want makers, you want revolutionaries, but having them be at odds on a cosmic scale is fucking stupid and not good. There are on the other hand many ways of being a massive self-serving dickhead, and nothing prevents massive self-serving dickheads from rational infighting.
Law and Chaos are not sides to the cosmic conflict. Formians breed like ants and just happen to live in places they find pleasing. Slaad spawn as described. Inevitables and chaos beasts spawn from planar material, although in the case of inevitables the planar material in question might just be giant automated self-reproducing factories.
allanlerouge wrote:5) Are the "major brand" of Outsider for each of the 9 "basic" Outer Planes (LB: Archon, NB: Guardinal, CB: Eladrin, CN: Slaad, CE: Demon, NE: Yugoloth, LE: Devil, LN: Modron, N: Rilmani) make any sense in regard to their respective planes ? Are there other brands for the 8 "intermediate" Outer Planes, like the Demodand ?
Do the 9 "basic" Outer Planes make sense at all? D&D Good is too conceptually small, and it can't carry 7 planes. Fuck, it can't even carry 3. Arcadia, Bytopia, Elysium and Arborea are the same and also fucking boring; they're what your starting village becomes when you've won the game. Really, there are more things to do on the Positive Energy Plane. Beastlands and Ysgard are stupid. One Good plane can be enough. Mount Celestia can just be a Law-infused geographical feature (and they're kind of Lawful Stupid-ish there to preserve planar balance and
literal personal integrity, which serves as a source of conflict).
D&D Evil is on the other hand varied, the Lower Planes are all fun, and you don't want to lose any of them (as well as Mechanus and Limbo).