Originally posted by LC1984
1. Is it possible to use sorcery in the underworld?
Yep. Sorcery can be used in any realm of existence.
2. Is it possible to bring items that were created in the underworld (crops, wood, etc.) into creation, or would these fade away in the sunlight?
I think the Underworld is a more interesting place if it offers treasures and resources that can be brought back into Creation, although certain things may still perish on exposure to sunlight.
3. Is there anything hindering a daring merchant to bring goods (like clothing), in greater quantities into the underworld, where garments normally fetch a much higher price? (besides the undead themselves)
They won't encounter any metaphysical laws that make it impossible, although such a venture is going to likely be extremely fraught with difficulties.
4. Can mortals sustain themselves with crops/fruits/water from the underworld? What about a tyrant lizard, is there enough game to hunt?
The flora and fauna of a shadowland offer the same amount of sustenance regardless of whether they're currently facing Creation or the Underworld. Past their boundaries, it may be possible to forage for food with a difficult Survival roll. I have trouble imagining their being enough readily accessible meat to maintain a tyrant lizard's diet; you'd likely need a creative solution like importing it from Creation, striking a deal with a ghost-merchant, or something else.
5. How much does the Immaculate Order know about the Underworld, or more precisely, a mortal priest/monk without a specialized background?
I think the big point is that the Underworld is the abode of those souls who cling to attachment to life instead of allowing themselves to be reincarnated in accordance with the proper order of things. Any more specific details, well, that's what Lore rolls are for.