Metamorphosis does not properly Summon a Fusion Monster, so you cannot use “Monster Reborn” on it afterwards.
Tributing a monster is a cost.
You can Tribute a face-down monster.
If you activated “Soul Exchange” targeting your opponent’s monster, if the monster is face-down, you cannot Tribute it as a cost for “Metamorphosis”.
You can Tribute a Monster Token (including a Sheep Token from “Scapegoat”).
If there is no monster with the same level as the Tributed monster in your Fusion Deck, then you cannot activate this card.
“Metamorphosis” vs. “A Legendary Ocean”: if you want to Special Summon a Level 6 Fusion Monster from your Fusion Deck, you have to Tribute a Level 7 WATER monster on the field because it is treated as a Level 6.
Netrep Rulings:
The player activating “Metamorphosis” must have a Fusion monster of the appropriate level that can be Special Summoned in his or her Fusion Deck.