I don't want to start any culture wars, but the style you attribute as "Cajun diatonic sound" is rooted in early Black Creole music at least as much as the work of the early Cajun greats. Amedee and Bois Sec Ardoin for sure, but also Bee Fontenot, and the Carriere brothers, etc.
If you believe some of the comments of the musicians on Arhoolie 307 Zydeco, "The Early Years", some of the basic zydeco licks go back to a generation before Boozoo.
I think it makes more sense to say that Clifton added blues and R&B into a diatonic zydeco genre, rather than to say that later zydeco players added Cajun diatonic to a blues/R&B idiom.
Not having grown up in Louisiana, and not being 90 years old, I am not a definitive source. But my ears tell me that there has been a lot of cross-pollination back and forth between Cajun and Creole players for decades.
Anyway, I'm mostly sharing my impressions of what was going on before that transition you were asking about.
Happy Holidays to all, and to all much excellent listening to the great history of Louisiana accordion music!
No, it was not Boozoo, John Delafose, etc. - they are "old school"
Whew, it would take to long to hit all the details here, but this is my take on it.
First, there is a bit of a problem with the term "Nouveau Zydeco". This term was created by Michael Tisserand (journalist and author), but it never caught on among the artists to whom it was most applicable (e.g., Beau Jocque, Keith Frank). I recall reading statements from one or more of these artists dissing the name pretty severely.
Noveau zydeco does not apply to Boozoo, John Delafose and other artists that were active in the 80's. They are more the "old school". But, they certainly had a very influential role on the next generation artists. In the 80's, both of them brought the single row diatonic accordion back to the forefront of zydeco, somewhat displacing what had been the more dominant piano accordion sound of Clifton Chenier, Buckwheat Zydeco, and others. My recollection was that Delafose is really the guy that got the single row going in zydeco, and Boozoo took it up seeing the success that John had (prior to that, Boozoo had been retired from music since the 1950's, and had played double and triple row diatonics, not the single row).
I think that the nouveau zydeco best applies to the "younger generation" bands that got going in the early to mid 90's. It is pretty fair to say that they followed more in the tradition of Boozoo than of Clifton (these two being the two foundational pillars of all zydeco).
I agree that Zydeco Force was one of the first to which that term would apply - they had many innovations like elaborate vocal harmonies, better production standards, more "upfront" rhythm section (bass and drums).
Next up would be Beau Jocque, who arguably had the biggest success outside the usual zydeco music circuit. I know many people who like Beau Jocque, but know very little else about zydeco.
Then of course there is Keith Frank. I think Beau Jocque beat him to the punch by a year or two, but you cannot underestimate Keith's HUGE success and influence.
There are lots of others that have come since, but I think the above three are the ones that mainly come to mind for the beginning days of nouveau zydeco.