Flashback timing works the same as its spell type , so a sorcery with flashback can only be played when you could normally play a sorcery
I'd call this blindingly beautiful
If you regenerate that attacker before damage is dealt, it will still be considered an attacker unless the regeneration effect itself triggers at the exact moment lethal damage would be dealt

Each Collector Booster will have the following contents: Collector Booster Info 4 Foil commons, common dual lands, or basic lands 2 Foil uncommons 1 Foil showcase basic land 1 Extended-art rare or mythic rare (45 rares, 8 mythic rares) 2 Foil rares or mythic rares, or foil extended-art rares or mythic rares 2 Showcase commons or uncommons, or a borderless card being previewed by Martin Juza on June 8 (5 commons, 6 uncommons) 1 Showcase or borderless rare or mythic rare (8 rares, 17 mythic rares including 5 versions of mythic rare Teferi) 1 Foil showcase common or uncommon, or a borderless card being previewed by Martin Juza on June 8 (5 commons, 6 uncommons) 1 Foil showcase or borderless card, any rarity (5 commons, 6 uncommons, 8 rares, 17 mythic rares including 5 versions of mythic rare Teferi) 1 Foil token In a pack of a Core Set 2021 Collector Booster, you are always getting a minimum of four rares or mythic rares, and about 45% of the time, you will get five