As for mithral, I'm honestly not sure just yet. Right now, I'd say it "counts as medium" means it counts as medium, including when it would be beneficial otherwise. However, logically that makes signifigantly less sense, and I hate that.
Yep, those were my thought. I personally dislike mechanics that require exceptions, but mithral isn't any less durable than steel. IIRC, it is stronger. But at the same time, exceptions suck.