I don't think the dodgy Manchester accent in Frasier was the fault of fault of Jane Leeves, who played Daphne. I think teh people making the show rejected the accurate Mancester accent she offered them in favour of sonmething they thought Americans would be more likely to understand. And she wasn't born in Surrey, but in Ilford in Essex (though they've changed the county boundaries now to make out it isn't).

................
how in the world soes rhythming slang get created and propagated? Most of those would be totally opaque to anyone who doesn't know it.

That was the general idea originally, and I think it'd still be seen as a bonus to have it baffle an outsider. A lot of the old expressions have settled down so no one thinks of them as rhyming slang these days. I mean, I doubt if many people describing someone as "a berk" are thinking of the term as an abbreviated version of "Berkshire Hunt".