The Fabled archetype that Fabled Raven belongs to gains advantage when their monsters are discarded via card effects. Some of them are able to revive themselves when discarded like Fabled Lurrie, The Fabled Ganashia, and Fabled Soulkius. Fabled Dyf revives Fabled monsters you discarded.
No it doesn't. It's part of the effect. (Which is pretty easy to determine given the "and if you do", since it's not exactly possible to fail to pay a cost.)
in my opinion, this card is one of many legendary synchro material ever made. it can also help xyz summoning in the future. you should get this card cz its effect is so helpful for swarming, otherwise it's so fragile to backrow.
one more thing, please keep in mind that synchro summoning is at its peak when there's 5 monster zones due to the swarming power of synchro summon. with just 3 monster zones they will limit the potential of synchro summon.
(most of great monster cards that used to be synchro materials are level 3 or lower)