As you will be moving, you will be required to notify the CAA of your new address. That invalidates the 61.75 anyway.
I am not sure you can add a Seaplane rating to a 61.75 but stand to be corrected on that front. If you make any changes to the 61.75 you will have to go through the full process again which will mean paying the money to the CAA waiting for the data to be released and visiting the FSDO.
You would be much better off just converting to an FAA PPL in the circumstances you state.
You can't bury your head in the sand regarding the house of cards scenario as it does rather seem to apply to you....