I found the rubber tree before I even went to Jerkbait island for the first time, so I disagree with that part. It was there, you just had to examine the coconut anytime you zent to the island (and it should attract your eyes, really) and you'd be well aware of its presence.
The Seahorse thing I didn't find hard either. It's not unusual for a line of dialogue to unlock new options (first you ask them to cooperate, then you can suggest how) and once I realised you could distract them it was pretty easy for me to figure out what to do (put something in the chest) and with what (the parrot, that would make noise).
However I did re-explore the whole forrest first since it looks like they come back from there after they bury the treasure. That's only after a while that I thought to go back to the 3 small islands (of course I went to the right one last. Stupid odds.)