Thanks for the reply.
As you will be moving, you will be required to notify the CAA of your new address. That invalidates the 61.75 anyway.
The address on my licence will remain valid for now (house I own in the UK), however changing address after July 1st would trigger an EASA licence which would indeed invalidate the piggyback.
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 say any changes to the 61.75 - the way I understand it, I have to go through the validation process if the underlying licence changes. Otherwise it would be impossible to add any ratings to the piggyback, and I know the IR is possible. Searched a bit and couldn't find any explicit exclusion of class ratings:
14 CFR 61.75
(c) Aircraft ratings issued. Aircraft ratings listed on a person's foreign pilot license, in addition to any issued after testing under the provisions of this part, may be placed on that person's U.S. pilot certificate for private pilot privileges only.
Again any clear written reference either way would be awesome.
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.
I'm not trying to escape the standalone, but my thought process is as follows:
a) due to visa issue in the middle-east my passport will be all over the place, not sure I have the time to organise TSA/M1 visa this year;
b) kind of silly to do a standalone private this year and then a standalone commercial next year, hence was trying to ride the piggyback train a bit longer;
c) apart from JB in Florida, which I'm not particularly attracted to, I don't see any part-141 seaplane schools, so even if I were to get a standalone this year it would mean attending two different schools.
Any suggestions welcome.