a "new" mechanic would be a special rule - but special rules can also consist of other things that aren't mechanics. Darkness woukd be a new mechanic and thus a special rule.
If its already explained in any official document, then there's no need to repeat the rule in your quest note.
I dont ever want to impose a word count maximum. I want quests to be readable easily, and accessible. I just want the quest map up top and notes on the bottom because that's how we expect to see the quest and play the quest. It should be normal convention, it should be expected. And I want users to choose the program they want to use based on look and feel and whether it can provide all they need for their quests. The programs are different by design, and that's important for the user to know.
Post processing is only allowed for heroscribe, and not if you are using the other tools - that's blatantly in the rules (not sure if that's part of what you're asking, but just so it's clear to others with similar questions). Some tools don't do what the others do, which is why they were all allowed this jam.
Everything not in the quest isn't judged. The itch page design, the homebrew cards, none of it matters for submission purposes. Go ham on cards...or don't make any at all. These honestly are for your flavor but aren't counted as part of the quest.
I hope that helps. Much of this I'll try to condense and make more explicit next year.