How badly are they marked and do you know when they were last painted? Normal painted walls are deemed to have a lifespan of 8 years, so if the walls were painted 7 years ago you would only be required to pay 1/8th of the cost of repainting the wall. Also, just because a part of the wall is defaced, you could argue that it only needs touching up and not repainting totally.

Also, your insurance will still make you pay a deductible unless you have also taken out the deductible insurance with the MV which will then see everything covered.

In any case, if you decide not to touch it up yourself and to face the music when you give the place back, you may want to get a MV rep to join you. I think it costs about SFr 120, but they will argue your case for you and make sure you are not paying more than you need to.