I have no idea which cave you are talking about or what diamond, but I usually use my Hero's Bow or my Clawshot to get to blue, yellow or red activation diamonds.
There isn't an Outset Island on Zelda Twilight Princess only in Zelda Wind Waker and other ones.
After beating twilight princess you don't get anything.
Google "Zelda twilight princess game cube walkthrough"
He is the Master Bomb Maker in Zelda Twilight Princess
defeat ganon
The last level on Zelda twilight princess is hyrule castle where you face the final boss ganondorf.
Twilight Princess
Zelda Twilight Princess
no answer possiblego
no
No.
No there isn't