Coinbase support is legendarily awful. This forum, reddit, twitter, medium and elsewhere are filled with stories of people submitting multiple tickets/emails/calls over several months and only ever getting automated replies that solve nothing.
Be that as it may, Coinbase is regulated by the US government and won't get away with keeping OP's coins if OP can prove what happened. They're not some shady, unregulated exchange with an unknown address like Yobit or one of those. So even though their customer support might suck ass, I have a feeling that OP can and will get his bitcoins back as long as he aggressively pursues this, which he definitely should.
Aren't there any lawyers on this forum? I don't know who OP could go to with this problem, but if Coinbase is stonewalling him I'm sure he's got options as far as help goes. I just don't know who.
That's it... I asked for my Bitcoins back and they sent me an email saying that I would receive without problems in less than 24 hours.
Whoops, I missed this followup post. OP, did you get your coins back yet or what?