I don't know a lot about how DMs Guild works, other than what you upload can't relate to any particular non-fat run d&d setting, and you get to use any d&d art packs they uploaded.
Are you allowed to compile and update old OOP materials on DMs Guild?
Are there any good existing faerunian lore based DMs Guild products you would recommend?
How does it work? What are you allowed or not allowed to make? How do the profits work?
I assume you're not allowed to take an old 2e source and simply update any mechanics to 5e,but are you allowed to write new sources that take their information from various source books and novels?
All those products have been written by CK scribes. You're allowed to write anything except fiction (although that's kind of up in the air at this point. Ed will be publishing some fiction there soon, but then, he's Ed). Idk if you can take a 2e book and just replace the mechanics with their 5e equivalent, but I guess not. You're allowed to use whatever lore has ever been released about the Realms, tho. As for profit, you get half of the money.
Mathematics is the art of giving the same name to different things.
More crunch and less lore, but I've got a decent amount of lore. Its usable throughout Faerun with some focus on Thay and Rashemen and the red wizards.