Is it obligatory to adhere to the wordings mentioned in the Abrahamic prayer when invoking blessings upon the Prophet (peace and blessings be upon him) during prostration, or is the matter flexible?

There is no specific formula that must be adhered to for sending blessings upon the Prophet, peace and blessings be upon him. It is preferable to adhere to the forms that have been narrated from him. It is permissible to send blessings upon him using any of the established forms. It is mentioned in Sunan al-Tirmidhi that the worshiper begins by praising Allah, then sends blessings upon the Prophet, peace and blessings be upon him, and then supplicates for whatever he wishes. Sheikh Ibn Baz was asked about reciting the Ibrahimiyyah prayer in prostration, and he replied that it is one of the means by which supplications are answered.