gorgeous guitar but the seller should offer a full refund, including initial shipping, plus pay for the return shipping. If the lack of packaging upon arrival is well documented/photographed, this is a no-brainer, and if you paid with Amex you should be pretty well protected.
alternatively Amex could help you negotiate a partial refund to keep the guitar as is, the way you got it. for me personally that number would be a minimum of $1,000. maybe $1,500.
If the seller is pursuing a shipping insurance claim, you will likely need to return the guitar to them, one way or another.
If the packaging is as bad as you say, that shipoing insurance claim could potentially get complicated (also in a different country), so be sure all bases are covered if and when you send it back.
While I admire your spirit and agree that looks more like fretboard separation than a gnarly headstock broken (and yes it is insanely beautiful)...I would hope you contacted your credit card company and froze the charges, started the dispute/investigation process. nothing to lose, u can always drop it.
i am with you on keeping it, at a price you agree to. what you currently have in your possession is not what you paid for.