20 thou isn't a light skim by any means, must have been bent like a banana or corroded badly!

depends on what you intend to do with it really. if you're looking to build a monster engine then i'd find a better head. if you're looking for something relatively normal power then you might get away with it. The seats shouldn't really cause a problem unless you're getting it very hot. I work on cylinder heads every day and can tell you that i've seen valve seats fall out of heads no matter how much they've been skimmed (sometimes they haven't at all). Other seats stay in just fine even when they're right against the head face. there's no rule to say that if the head is a little thin then they'll fall out, but it may be worth getting a machine shop to stick them in a little deeper if they think they need it. I like to have .250" deep inserts in aluminium as a rule, deeper if space allows.