The Magical Lamp was added to the item pool to ensure Diablos is available in the early-game, even if the player missed talking to Cid earlier (normally, the only other option of getting more is through Chocobo World). There is currently no other use for additional Magical Lamps, besides selling it for 500 Gil each, but I'm thinking of more usages, like making it refine into something at least.
The new shop in Deling City primarly serves as an alternate and quick way of getting level 2 elemental spells and Cura, for those who haven't found these through other means yet (the end of disc 1 is the absolute latest the player should get them). Later in the game, more items will be available for this shop using Tonberry's Familiar Ability. I would love to add even more unique shops to the game, but it's probably not that easy, as there is only one dummied out shop in the game data that can be made into basically anything without touching the already existing shops.
As for the first Edea battle, yeah, it's pretty much the same as in vanilla. I didn't want to change it into a must-win battle, as the party ends up being in trouble afterwards anyway. Still, there might be updates to at least Edea's attack pattern and AI in a future update, like having her use weaker spells at the beginning, and switching to stronger ones as the battle progresses.
Anyway, thanks for sharing your impressions. I'll keep your Deling City sidequest idea in mind. Thought about that couple earlier too, and making something Triple Triad-related out of it could be really interesting.
The reward for joining the Garden Festival Committee is one Luck Up if you join during the party. If you are already a member, you get a Life Ring on top of that. So you didn't miss too much, I guess. It's ok for an early sidequest reward, but nothing more.