I might be getting too many points (I am hoping I will not bring "account regularization" upon me), but usually what happens to me is that I get the regular points, then on top of them, or already included, any promotional terms, and only on top of that comes the "points accelerator", proportionally to ALL points earned for a stay.
So I would have expected it to work like: regular points (2 points for every Euro), then doubled, and THEN the "accelerating factor" applied.
Mind you, I never stayed at a Mercure Privilege room. Given the mess the Mercure chain is, I would never pay anything more than the base rate (and avoid Mercures in general).