Tyres are covered by the tyre manufacturer warranty. Suggest you contact the tyre manufacturer and ask what they suggest.

Downside is you have to buy new tyres before you return the old ones unless you have another set,

Last year a set of 3 year old Michelin Energy on the wifes car showed small cracks on the outer sidewalls. The tyres still had 5mm on the fornts and 6mm on the rears thus not 1/2 worn. When I fitted the winter wheels the tyres were returned and we got a £195 cheque from Michelin for the remaining tread.

If its not wheel balance its got to be the tyres, I had a pair of new Goodyears on a Citroen XM when I bought it, the car always pulled to the offside - until I changed the tyres. You possible have ply seperation, one way you an check is to jack one wheel up just clear of the road and run the engine, observe the gap between the tyre and road as the wheel rotates, you may find that it is varying because the tyre is out of round.