No. Overload and flashback are both alternate costs. If you flash something back, you must do so for its flashback cost. You may only pay one alternate cost when you cast a spell, so flashing something back takes that up and you won't be able to use the overload cost.
If overload was an additional cost, then you could flash the spell back for its mana cost and then pay its overload cost to give it overload (much like you could kick a flashback spell if it had a kicker).
I would have to think so. Overload does not say from hand. Snapcaster allows you to flashback, which the wording used is cast. Overload also uses the word cast.
No, it says you may flash it back only for its flashback cost which is equal to its mana cost.
No, it dosnt say ONLY. It says the card GAINS flashback and that the cost to flashback is its mana cost. It does not say that you have to pay the mana cost as opposed to some other form of cost.
Epochalyptik's answer is correct. Both Overload and Flashback are alternative costs that replace a spell's normal mana cost. If you apply one of them, then there's no normal mana cost for the other one to replace, so the other one can't be applied.
@ChaosOrb: If a spell has a flashback cost, you can't pay some other cost to flash it back. That's like trying to pay for a kicker cost by paying the spell's mana cost again.
Think of it this way. How can you play a spell from a graveyard? One way is Flashback, which reminder text reads, "(You may cast this card from your graveyard for its flashback cost. Then exile it.)". You need to pay the flashback cost in order to play it from your grave. If you try to play it through the Overload cost, how is it coming from your grave? The Overload cost does not allow it to be played from a graveyard.