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Hello. In order to deactivate cross save, you must be able to authenticate all linked platforms. Since you have already contacted PS Support, then there is nothing you can do to unlink your accounts. Cross save can not be deactivated until you gain access to your account.
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For security reasons accounts can only be linked and unlinked from Bungie profiles using valid platform log in credentials. Bungie does not have access to platform log in credentials and thus can't link or unlink accounts on behalf of players. You'll need to continue working with Sony to recover the log in details. Otherwise you are stuck with this cross save set up indefinitely.