I would like to point out, to avoid any misunderstandings, that none of the Spanish lawyers who give advise in this forum are paid by Mark. We are not his “legal team”. We are independent.
Back to your query, you are right.
Only if in the contract there is a clause in which the developer agrees for you to assign the property to a third party prior to completion (normally at a given fee) can this be done.
As for enforcing the Bank Guarantee on the grounds of late delivery (one year) I doubt you will be able to do this because that is not it’s purpose.
I’m afraid you will have to complete like it or not or else litigate and try to pull out of the contract alleging that the property is being delivered substantially late, and the latter will be expensive as well. In any case it is hard to give an opinion without having read the wording of your contract.