I think the problem with the Great Vowel Shift is that it's mostly done by the late 18th century, and the earliest it could affect the American dialect is about half way through in the 1600's, at which point it is fairly recognizable.
I think any such declaration might be very much latter, probably consisting of part of a public school reform or something like that in the 1890's at the earliest, but otherwise, this might work.
So, USA and Britain stay enemies longer into the 19th century, and the USA turns to France instead. German influx happens with the liberal revolutions and people migrating. American English eventually turns unintelligible to British ears, and congress declares it its own language by latter parts of the 19th century.
I think any such declaration might be very much latter, probably consisting of part of a public school reform or something like that in the 1890's at the earliest, but otherwise, this might work.
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