- Anybody can ask a question
- Anybody can answer
- The best answers are voted up and rise to the top
Yes. The only requirements for Flashback are that the card is in your Graveyard, you are able to pay the alternate Flashback cost, and you are capable of casting the Flashback spell at that particular time (you have priority, if the Flashback spell is a Sorcery then it is you turn's first or second Main Phase and the stack is empty, etc.).
Keep in mind that if your opponent Cancels your Army of the Damned spell, that you must wait until Cancel to resolve before your spell will be in the graveyard. If it was Dissipated, it would never hit the graveyard at all.
Yes. Flashback simply lets you cast the spell from your graveyard, it does not matter how the spell got there.