1] If you opened the door in the lower guest garden, where the coin fountain is, you may not have enough halla statues for the rest as that door requires three halla statues. If you want the Celine and Briala ending, you must have the elven locket. So you only have enough to open
the Locked Door on the Upper Balcony of the Guest Gardens (Upper Gardens) (1 Statue)
the door to the vault where you pick up the elven locket (3 statues)
Celene's Private Bedroom in the Royal Quarters (5 Statues)
2) If you want to get 30/30, you really must make sure you get information when you eavesdrop. If the inquisitor said it's nothing or boring, that means you didn't get the scandalous gossip. You have to step away from the area for a while and listen in again until you get it.
Too, you have to revisit areas you have been to before in the 2nd (after talking to Morrigan) and 3rd (after the dance) parts. The Grand Library especially, to check for papers that will appear there. Also check the lower corridors of the vestibule, all around the ballroom. Don't bother with the servants' quarters and the gardens attached to that section, nothing will appear there. If you get all 30/30 (you must hand them in to Leliana before you meet Florianne's men), you have more leverage.
3] You have to talk to Celine's handmaidens, the three ladies and show them the elven locket. Celine will talk to the Inq. Pick all the first choice replies that implies that she's still in love with Briala. Next, talk to Briala, show her the elven locket. Again, pick all the first choice replies, implying Celine still loves her. You must have evidence on Gaspard. You get this by telling the tied up Chevaliar on Celine's bed to testify against Gaspard.
You'll be able to implicate Gaspard and reconcile Celine and Briala in the confrontation.