This may not be the way the system gets implemented but, in previous posts, it has been mentioned that players will only be able to make and/or hold one of each legendary card at a time : they will need to cast/dispose of it first before they can start to make/obtain another. I mention the and/or's because the cards might also be traded in the card markets and its felt these cards are way too powerful for any one player to be holding or using multiples of them.
Of course, this will not prevent a team from combining to use various legendary cards against a single target : but then the prize has to then be worth that cost. That's the balance that must be sought in designing the cost of making and use, together with the reward gained, of these cards.
It's an intriguing concept and I do like the thought that has gone into your proposal.
My thoughts :
I am against the idea of a 'single cast' ship-stealer, particularly for the MoW : its a trophy ship and should remain both hard to acquire and to lose (unless the owner is careless) : We do have means to steal other classes of ships by a single-card, (Commandeer and Tidal Wave), but again, it relies on the owner being careless and leaving them out-of-fleet : I'm not in favor of being able to steal a ship while it is in fleet though.
I am in favor of being able to destroy ships in fleet : that is the key to stealing high-value ships from in-fleet and perhaps that is where you should look to modifying this proposal. Currently, that needs 10 Call Leviathan (for a level 10 target) and 80 turns : given the turn cap which does act to restrict such raids, then there is a place for a Legendary card which fulfils that function. The making costs thus need to reflect the cost of using 10 Levi and those turns but not too much more : that way, the raider can 'preload' this card and save on the immediate use cost.
I like the idea of a 'scraps bonus' and officer recruitment chance arising from the cards use but its not something I would demand when using such a card : the point is the overall raid objectives, the scraps/officer bonus would be some jam on top of that cake. So yes, include it but it would not be the prime reason to use the card.
Lastly : please dont be disheartened by criticism or adverse comments : you have done well to present a clear and cogent proposal with some real 'meat' on the bones that can be debated over.