The simplest problem, and one I've articulated before, is that true denizens of Carceri are the kind of person who would stop right before the finish line so they could turn around to flash the middle finger to everyone else. Carceri is the plane of the monologuing villain who just has to let slip his master plan in an attempt to make the hero even infinitesimally more miserable. It's the plane of sour grapes. Why wouldn't they go after the Mercykillers' portal, or any other known and stable one? Because it would mean accepting on some level the help of others was necessary to do what you couldn't alone, and everyone else in the multiverse is a schmuck. Not only are they all schmucks, but it's also vitally important that you let them know they are all schmucks. Gods forbid someone be allowed tobe wrong on the Internetthink they were in any way needed, or in any way more powerful than you.
Lolth controls a couple of worlds where she is the only deity. There was a world depicted in a Dungeon magazine adventure ruled by a trio of deities, which is close but not quite. Jakandor might be closest; the Charonti used to worship a god of death and rebirth called Thanhotep (some few still do), while their opponents the Knorr ubiquitously follow the War Mother. They have some subordinate beliefs (hearth gods, various spirits) but only one unifying deity.
None immediately spring to mind; I'm assuming Ravenloft doesn't count, considering.2) Aside from the Spirit Realm of Kara Tur, the Gray and Black of Athas, and the Shadow World of Aebrynis (and kinda sorta Krynn's Ethereal Sea), are there other settings out there that have replacements for or variations on the local Ethereal and/or Shadow Planes?
Bastion of Broken Souls is basically the source for information on preincarnate souls, which spring forth from special places in the Positive Energy Plane.a) The fate of the soul after death is fairly well-defined in various materials, but the origin of souls is much less so. Aside from the information on the Wellspring in MoI and the soul fonts in Bastion of Broken Souls, is there any other information on how souls are made, how creatures are ensouled, etc.?
We've never been given the answer in any detail, but the heart of a soul is pure positive energy. As it changes from positive energy into a soul, it acquires what one might call an "address," a unique serial number demarcating its place in the universe as a thing separate from the energy that birthed it, which gives it an identity to the Astral conduits that will transfer it where it belongs. This process of becoming something distinctive is when a soul becomes a soul, and as its energy becomes more than the sum of its parts, it transforms from simple positive energy into the blend of essentia and positive energy that a soul is, linked to its Astral address and its temporal thread.b) What are souls made of? Just pure positive energy (and, if one is using MoI, presumably a bit of essentia), or N parts positive energy to M parts belief-stuff, or...?