Why not offer to pay for the opposite side at trade price. I did this successfully when had similar situation on My 370z at a main dealers. That was with different wear rate let alone tyre mix.
Cont 5 went on today. Mercedes service man assured me that these 2 are compatible... The other front has less than 1000 miles on it so there was no need to change other than to ensure matching pair. I will report back and if I notice anything then I will change it. If I understeer into the undergrowth then.......... Too late!