A. What will decrease Chiepoo's shop "level"?
You actually only need to complete 1 quest, "X & Co's Revenge", to drop Chiepoo from his top status to absolute dirt status.
However, completing other X & Co quests will also reduce Chiepoo's shop level, but I believe the Revenge quest is the only one that will drop him to the lowest.
I think there's a proper time to do this in order to acquire a rare Hunter's Orb, Champion's Orb, and Silence Orb, which only the poorest Chiepoo shop level will sell. After completing the Small Border Village battle, the Small Border Village will offer pretty much everything Chiepoo's best shop had except for Platinum Mail and Platinum Gauntlets. These items are sold in Graska, which you gain access to after the first battle in the Imperial City of Graska.
B. How to unlock the "Savage Sprout Attack" quest:
- Successfully complete the dispatch quest "Retrieve Our Stolen Cargo" (dispatched character needs an ATK of at least 80)
- Then complete "Savage Sprout Seeds Wanted" (hand in 1 Savage Sprout Seed)
- "Product Recall" will appear. Purposely fail "Product Recall" by assigning any character except Kate or a Nay-Kobold
- Once Product Recall's failure result is reported, the "Savage Sprout Attack" quest will appear.