Easy to get a CoC from VAG Europe and free of charge, just tell them that you intend reregistering it in another country, dont be specific because within the EU you would not need to.
Otherwise go to the Drire with your Carte Grise or the Type Mines numéro and ask for a copy of the barré rouge, there might be a slight admin charge for it, Barré Rouge actually means the EU CoC for your vehicle which has a red band across it preventing someone else using it to support a registration application for another vehicle that they cant get one for, maybe a Yeti variant not homolgated in Europe like the Chinese built ones or a modified fire service or military version.
That will tell you if your wheel and tyre combination was homologated for your vehicle but Skoda France have already told you in very specific terms that it wasn't.
If you want the CoC to back up a claim for losses you really are pi55ing in the wind, you have not suffered any, Roots post above is bang on.
The incorrect tyres have long gone, you got the full service life out of them, had you just bought a set of new ones in the same size I could understand your frustration at having to replace them but its 6 years since the selling dealer fitted the wrong tyres (if the CoC says they are) and when you renewed them it was both yours and the tyre depots responsability to ensure that they were correct ones homologated for the vehicle not the dealer who sold you the car 6 years ago.
You have suffered no losses, I hope you get a COC and find by a remote chance that VAG France were talking rubbish, that the tyres you wish to drive on are in fact homologated for your vehicle, I fully understand the difference in ride, I feel it myself on some of the winter wheels.
It would be a shame to have to change the vehicle to get the ride quality the chassis deserves but Greenline models were compromised in many ways to achieve the lower CO2 rating and also to fit into the lower Puissance Fiscale (old school horsepower rating) in France, my vehicle has a 6 speed box but those supplied to France had a 5 speed box, it was registered in France by me (RHD UK import) as something it never was the French version.
I take a different view to the god fearing French and know that the use of the word "illegal" is BS, its simply a non conformity, they would all have you believe that your insurance will be invalid, that they wont pay out on a claim, its all BS swallowed by 99% of the French because the education system programs them to never question what they have been told. Sure if you fitted solid rubber tyres on wooden spoked wheels from a veteran car and you lost control on a bend and hit another vehicle the non conforming modification will have caused the accident and they could say it was direct and known negligence but another tyre size homologated for another Yeti with the right speed and load ratings, no way will that ever be a problem except at CT time.
I will need to replace my tyres soon and will try to find one with the highest most compliant sidewall, as long as the speed and load rating are OK I dont give a Four X whether they are homologated and will simply put the compliant winter wheels on for CT.
My sincere advice to you is find a solution for tyres which will give a good ride without having to sell the car and forget suing the selling dealer, you are setting yourself up for no end of angst for zero return.
When VAG homologate a range of tyres for a vehicle variant it is so they can offer option packages for each country, the UK seems to love skinny rubber bands on massive wheels despite the crap roads, other countries may require compliant tyres and more ground clearance, they homologate all the ones that might be fitted in the factory and also in case the factory of one supplier of wheels for instance burns to the ground, they can (and sometimes do) switch to an alternative.
In the case of the Greenline models very few tyres met the low rolling resistance criteria for the CO2 rating, ther may even be only one size homologated, when you buy a vehicle like that you accept that in the future you will have less choice of tyres etc, when things like run flat or Denovo tyres cease production then people just fit whatever else is available, they probably did so from the first replacement, this is not so easy in France with its rigid beaurocracy.