Pay Money Bags 500 Gems to open door
Don't have the money? - Go back to the building and jump across to left of the arch and aim for the bridge/dock. Make sure Sparx is at full health.

Egg2 Jump to the island Michael

Shiny the Firefly
Egg1 Find Shiny the Firefly Thelonious

After you get to the exit and you get the tea lamp there you can exit out other side of the lantern and glide to a few tree tops. One of those has the key on it.

Egg6 Across the treetops Frank

Skill Point 8 Destroy all piranha signs

Bubba the Firefly - First light the 4 tea lamps and the door will open

Spyro Challenge Portal - Sleepy Head

Sleepy Head has 2 attacks, first he throws a bomb at you, then 3 gators. Charge the bombs back at him before they explode to hurt him. Flame or charge the gators.

Egg5 Defeat sleepy head. Herbi

Sheila Challenge Portal - Twin Bombers

Basho the Firefly

Egg3 Escort the twins I. Peggy The first guy stays on the path

Buson the Firefly

Egg4 Escort the twins II. Michele The second guy has his own way

I would have to say the most likely place for missed gems in Spooky Swamp is in Sheila's part where you help the twins.

There are 4 walls that she can kick, 3 of them are way up high so you have to use her double jump.

1. Left and behind of where you meet the twins, up high
2. & 3. There are two rooms to the left off the main path. High on the back walls of each is a breakable wall
4. behind the eggs, a section of wall that doesn't look breakable, but it is. This one is at ground level