Amex certainly has somewhat limited acceptance, and they are addressing that - too slowly for many of us, but in the US at least they have made big strides with the OptBlue program. It does become more hit and miss outside of the US. One thing I would say is that most of us don't consider the Platinum card to be a daily driver. It's for the perks and protection more than anything. Using it for big-ticket travel purchases (hotels, airfare, auto rentals) and higher end dining, you'll find almost universal acceptance in most of the world.
Also +1 on the GE taking forever to do; my family actually ended up doing our interviews outside of the US as there were no Boston appointments available at all and I know many other locations have their earliest appointment 6 months or longer out. The good news is that you can always schedule any available appointment to hold your place for GE, then if one becomes available at your local airport sooner or you find yourself traveling to (or though, if the layover is long enough) an airport that has available appointments, you can get it done there.
That's a very good point about regions, Irish. I recall looking and seeing that I could have had near-immediate appointments if I could find a reason to drive to Newark, but for Boston there were none available and other New England locations were 6 months or more out if they were available at all.
Sorry to hear about your overall experience. It's well known that Amex isn't accepted overseas... sounds like you weren't prepared for that! I have a million and one Amex cards and mainly used my Quicksilver the several times I've been in Europe this year as I knew what it was in advance. Most purchases for bonus categories on the Amex has to be 'US' related anyway, so there is no real reason to using an Amex overseas. Unless you're buying airfare, it's really for the perks and that's it. Sounds like you have way better cards in your wallet for overseas purchases. If you're looking for a 'overseas luxury travel card' then maybe consider the CSR as I'm sure it's Visa/MC based.
Amex $200 airline credit is stupidly set up (this is also known lol). It should be more flexible like other cards... but it isn't for whatever reason. I just get a $500 in Delta gift cards off of all my PLAT's and PRG. It's the best way to go. No reason in making 'accidental' charges on purpose as it seems like you have pretty much all your expenses covered!
As for the decline... not sure about that one. Something must have triggered in their system. Sorry for that experience as a charge like that should be easily accepted and can be a tad bit embarassing for some people. Amex doesn't require you to contact them. They told me they were aware of me going overseas (bought my tickets via Amex Travel (50% point rebate yeah! lol)). The few times I did use it (which was probably 3 times) it went through without an issue.
If I were in your shoes, depends. Most of the points you hit on are well known so I personally wouldn't close it. But if you want a card you need to use over seas, use the Visa/MC's that you have and avoid Amex (and Discover). Get a CSR if you want a travel card. But as Irish stated, don't get the bonus and then close the card. Amex will hunt you down 
Good luck with your decision!
My Global Entry took about a month for me and a month for my wife. I was a little upset that I had to drive 2hrs TWICE to what was a 5 minute interview. Could have did that through Skipe! lol
@UpperNwGuy wrote:Most of the commenrary on this post has focused on the general lack of Amex acceptance throughout Spain. However, OP also expressed frustration with his card being declined by Amex at a merchant that did accept Amex. I'm wondering if OP had filed a travel notification with Amex before leaving on his trip.
Amex actually stopped doing travel notifications quite a while ago. Sorry, I meant to mention that in my earlier post.
The decline is odd since my experience has always been approve now, inquire later. Although this is a new card and my charge cards have been open for years so that may factor in a lot.