Now, can I use this to respond to a flashback or scavange? No. Flashback and Scavenge both remove the card from the graveyard, where you would "target" the card with your spell. Without a valid target, you cannot cast Cremate.
702.32a Flashback appears on some instants and sorceries. It represents two static abilities: one that functions while the card is in a player’s graveyard ... . “Flashback [cost]” means “You may cast this card from your graveyard by paying [cost] rather than paying its mana cost” ... . Casting a spell using its flashback ability follows the rules for paying alternative costs in rules 601.2b and 601.2e–g.
601.2. To cast a spell is to take it from where it is (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. ...
702.95a Scavenge is an activated ability that functions only while the card with scavenge is in a graveyard. “Scavenge [cost]” means “[Cost], Exile this card from your graveyard: Put a number of +1/+1 counters equal to the power of the card you exiled on target creature. Activate this ability only any time you could cast a sorcery.”
Because it moves on to the stack, is it still in the graveyard for me to exile? And even if it was, would the effect be countered? No. Cards and other objects cannot be in two places at the same time. If the card is on the stack, it is not in the graveyard for you to exile.
If the card was in the graveyard, and for instance, Scavenge read like this (which it doesn't):
Scavenge [cost] means "[Cost]: Exile this card from your graveyard. Put a number of +1/+1 counters equal to the power of this card on target creature. Activate this ability only any time you could cast a sorcery."
The card would use Last Know Information, and the effect wouldn't be countered.
Last Known Information - Information about an object that’s no longer in the zone it’s expected to be in, or information about a player that’s no longer in the game. This information captures that object’s last existence in that zone or that player’s last existence in the game. See rules 112.7a, 608.2b, 608.2g, and 800.4g.
If perhaps the ability were worded similar to Standstill, with an "if you do," then the effect would be countered because Scavenge didn't exile the card.
Scavenge [cost] means "[Cost]: Exile this card from your graveyard. If you do, put a number of +1/+1 counters equal to the power of this card on target creature. Activate this ability only any time you could cast a sorcery."