Patagia and weird flight interaction

Yop'
I heard you like nosferatus, so i made this deck with Patagia, flaps allowing limited flight.

" +1 stealth action.
Put this card on the acting Nosferatu. This vampire gets an optional maneuver each combat. Once each combat, this vampire may burn 1 blood to gain flight [FLIGHT] for the remainder of the round. A vampire may have only 1 Patagia card. "

As worded, the cards says "i can burn 1 blood anytime".

Q : I am bleeding my prey like a boss when suddenly a wild potence prince appears. Combat ensues. I manoeuver to long using my patagia. The opponent chooses Earthshock as his strike.
( text : " Strike: strength ranged damage. This strike cannot be dodged. Not usable against a minion with flight [FLIGHT]." )
Is it too late to gain flight at this point ? (I guess not)
Does it cancel opponent's strike ? (I'm not sure)

It is too late to gain flight after the strike has been announced. Earth Shock cannot be announced as a strike against a minion with flight, it is irrelevant if the minion gains flight somewhere between announcement and resolution as the strike has already been played.

I wouldn't say it's too late to get flight (for example to be able to play Circle) but it won't affect the current strike resolution, the Earth Shock is already played.

I am not sure. Is "usable" different than "playable"?

I don't think you're allowed to play a strike you're not allowed to use, e.g Taste of Death at close range. If the strike is rendered unusable after being played that's another story though. Compare immortal grapple 'Strikes that are not hand strikes may not be used this round (by either combatant)'. It seems like burning a blood for flight would make the Earth Shock fizzle (but not canceled).

I don't see any restrictions against burning a blood for flight at that point to answer question one.

That was my impression as well, Louhi. The immortal grapple example is excellent; you can't declare any invalid strikes after it's played, and any invalid strikes already played have no effect.