No, Elesh Norn's static abilities have no effect on the battlefield in that case.
Bringing Elesh Norn into play with Flash and deciding whether or not to sacrifice it is all part of Flash's resolution. Killing creatures with her -2/-2 would require state-based effects to be checked, but that only happens after Flash has finished resolving, at which point Eles Norn is already gone.
If you sacrifice a creature brought into play with the spell Flash, basically all you get are "enters the battlefield" and "leaves the battlefield" effects.
No, since no player receives priority between Elesh Norn entering and leaving the battlefield during the resolution of Flash. State-based actions that would place creatures with 0 toughness in their owners graveyards, are not checked until just before a player would receive priority. Rule 704.4 gives a nice example of a creature that has 0 toughness temporarily, that isn't sent to the graveyard, because its toughness is greater than 0 the next time it would be checked.
116.5. Each time a player would get priority, the game first performs all applicable state-based actions as a single event (see rule 704, “State-Based Actions”), ...
704.4. Unlike triggered abilities, state-based actions pay no attention to what happens during the resolution of a spell or ability.
Example: A player controls a creature with the ability “This creature’s power and toughness are each equal to the number of cards in your hand” and casts a spell whose effect is “Discard your hand, then draw seven cards.” The creature will temporarily have toughness 0 in the middle of the spell’s resolution but will be back up to toughness 7 when the spell finishes resolving. Thus the creature will survive when state-based actions are checked. In contrast, an ability that triggers when the player has no cards in hand goes on the stack after the spell resolves, because its trigger event happened during resolution.
704.5f If a creature has toughness 0 or less, it’s put into its owner’s graveyard. Regeneration can’t replace this event.