From memory, they are painted or coated suggesting magnesium.
Im 99.99% sure they are not magnesium. Too expensive and difficult a metal to use on such a routine part.
I believe they are aluminium that is powder coated. Where corrosion gets under the powder coating, the aluminium turns to a powdery oxide that could suggest that its magnesium, but it's not. Your SLC covers will be anodised.
I've sold my 190E so can't look at the cover. The spacer system you describe should surely stop warping of 70thou. I'm not arguing with you, it just doesn't make sense the way you put it!