answering to my own post some months later ;D
Digging into the Flight International archive and other sources, it seems that the Mercure 200 very nearly missed the "A320" train in 1976.
1974 the "group of six" is born: HSA and BAC, Fokker, Aerospatiale and a couple of German companies.
early 1975 Dassault join the team with their Mercure 200: a CFM56 powered Mercure for Air France, with longer range.
Competitors are: BAC 3-11 and Trident. But also Aerospatiale (SNIAS) A-200.
Aerospatiale hates Dassault, notably the Mercure. For good reason: De Gaulle iron-fisted a rule: to Aerospatiale, airliners and helicopters; to Dassault, combat aircrafts.
Yet Aerospatiale tried to seduce Vought for licence-build A-7 Corsair II in Toulouse, they also insisted on trainer (against the Alpha Jet) only to be rebuked by Messmer, Pompidou and other gaullists.
Yet at the same time, Dassault, who had been told NOT to make airliners, screwed Aerospatiale building the Mercure, only to fail miserably.
Meanwhile the CFM56 venture between SNECMA and G.E got a turbofan running by 1974... and no aircraft for it, not before 1979 and the DC-8 / KC-135 re-engining.
On top of that, Aerospatiale is winding down Concorde production, but Airbus A300 is DOA until Frank Borman and Eastern Airlines, in 1978.
so Aerospatiale Toulouse plant is mostly empty.
In the end, on paper, what makes ton of sense is Dassault teaming with Aerospatiale to build a CFM56 powered Mercure - the 200.
That would kill three birds with one stone: the Mercure, the CFM56, and Aerospatiale.
What's more, Aerospatiale is part of the group of six... and Dassault, well aware of all the above, rush to join that group.
By early 1976 - job done. Even the British, even Fokker agree that the Mercure 200 make sense. What's more, that group of seven link to Airbus for a second project that later become... the A310 !
So all boxes are checked: Aerospatiale, group of seven, CFM56, Mercure, Airbus. By spring 1976, the group of seven is willing to adopt the Mercure 200 and link it to Airbus.
...and there it goes down the drain.
Because Dassault and Aerospatiale hate each others, in summer 1976 Dassault look toward... McDonnell Douglas. Then just to piss them off, Aerospatiale take the absurd step to study the A310 with Boeing! With the A.200.
What is very damaging is that Dassault went to MDD with full blessing of the French government, and this pissed off the group of seven.
The Dassault - MDD bird, Mercure 200, become the ASMR, only for Douglas to screw Dassault and develop the MD-80 with improved JT-8D.
In 1977 the French government order Dassault to join back the group of six... which now has become J.E.T. Within it, the Aerospatiale A.200 has slowly started to morphe into... the A320.
Dassault nonetheless join J.E.T... only for Aerospatiale to scorn them for six month until they leave the boat. Ding dong, the Mercure is dead, for good.
Now had Dassault and the French government NOT embraced MDD, then the Mercure 200 could have become an early A320. No FBW, for sure, but CFM56 and HUD and stretched variants up to 80 tons, same dimensions and diameter as the A318 / 19 / 20.
what's more, in the group of seven were Fokker and Avro. Later those two developped the F.29, F-70 and F-100 and of course the Bae 146.
And both sunk 20 years later, to eternal regrets.
On top of that, MDD did to Fokker F.29 what they did to the Mercure. They brought back the ASMR, with Fokker, in 1979, and same result. MD-90 this time.
It is amazing how MDD fooled Europe, twice, with a paper aircraft that was killed, both time, for a revamped DC-9.
At least that strategy finally blew into MDD face when the A320 ate the MD-80 series for dinner, in the 90's.
In an ideal world, Fokker would have become Airbus-bis with the F-27 and F-28, the later eventually stretched to link with the Mercure 200. Then, Airbus A310 and A300.
Of course ATR would never exists in the first place (1983) since it would duplicate the Fokker F-27 and F-50.