Flow

You can play it only when its existing timing restrictions allow.

Flow lets you play the spell from your trash for its Flow cost, but it changes no other permissions or timing restrictions.^{[829.1.b][829.1.b.2]} For example, Flow does not grant a spell Reaction or let you play an Action outside its normal timing.

It is banished unless another replacement effect changes that result.

Countering clears the spell from the chain and would normally put it in the trash.^{[425.1.a][425.1.a.1]} However, playing a spell using Flow creates a delayed replacement effect that banishes it if it would leave the chain after becoming finalized, unless its own execution instructed it to leave.^[829.1.b.1] Being countered meets those conditions, so the Flow replacement effect banishes the spell instead of letting it go to the trash.

Yes, provided the spell's printed cost meets Defy's restrictions.

Defy checks the cost of its target, and effects that determine a card's cost always use its printed or copied cost rather than the amount actually paid.^[206] A Flow cost is an alternate cost that replaces the spell's base cost during finalization,^[829.1.c.1] but it does not change the printed cost that Defy checks.

It is banished rather than returned to its owner's hand.

Abandon replaces putting the countered spell in the trash with returning it to its owner's hand, while Flow replaces the spell leaving the chain with banishing it.^{[425.1.a.1][829.1.b.1]} Because both replacement effects apply to the affected spell's departure from the chain, that spell's controller chooses their order.^[372]

The result is the same in either order. If Flow applies first, the spell is banished instead of going to the trash, so Abandon no longer has a qualifying event to replace. If Abandon applies first, it still replaces the original event with the spell leaving the chain for its owner's hand, so Flow applies to that replacement event and banishes the spell instead.^[370.2]