Execllent questions!
1. As long as you are describing exactly what is going into the room and where, and what is being taken away, these types of changes are fine, as long as you have room in your quest notes - the clarity of this explanation and how many steps it takes may affect accessibility rating.
2. Iconography is limited to the game system and chosen expansion, and letters/numbers (native icons in HQuestBuilder); to avoid bending the rule about only one expansion, I would use one of these unused letter/numbers or place a quest notes by the affected door.
3. If the assets needed can be covered by the First Light box, then First Light should be the game system version chosen.
Before finalizing the jam page, I forgot to take inventory and see if FL was actually missing anything present in the Game System. I actually need to simplify this language on the jam page.
(Edit: fixed typos)