To find Riku in Monstro you need to have found and equipped the High Jump ability. Using the High Jump ability you will be able to jump across some platforms in Monstro's mouth. This will let you reach a door you couldn't reach before and should eventually lead you to Riku.
Go beat Monstro. It is kind of easy. Just find Chamber 4 and then follow Riku.
You'll need to get High Jump in the Treasure Chest next to Gepetto. Once you got it find a board that you can jump the the highest ledge. I couldn't really describe on where to go. But maybe you'll find it.
Yes, monstro is in kingdom hearts, and he can be really confusing. just go in the door that riku went through (watch the cutscene) and everything else is pretty straight forward, just don't try and jump any gaps.
There isn't a keyhole in Monstro. All you have to do is beat the Parasite Cage twice. Once in the easiest part where you just enter right in, and the second time where you have to jump a couple of times where Riku was.
Yes, they do find Riku at The World That Never Was.
you go to the level that is monstro.
The Keyhole in Hollow Bastion is in the Grand Hall where you fight Riku/Ansem, or where you fight Riku the second time.
This isn't a location in Monstro City. There is the Moshi Fun Park though, which you'll find towards the top left corner of the Monstro City map.
You can find a red banana in Monstro City by the ice scream shop.
You'll find items to buy at the shops in Monstro City.
You'll find this shop on Main Street in Monstro City.
Captain Buck's ship, the CloudyCloth Clipper, travels to find items to bring back to Monstro City. When the CloudyCloth Clipper returns, you can find it at The Port in Monstro City if you are a paid Moshi Member.