A few answers to your questions.
1. You CANNOT combine the Amex platinum OBC with the referral coupon for $400. Obviously the latter gives you a hundred dollars more so go for that.
2. Most of the included alcohol are top shelf brands. You can get anything from Johnnie Walker black to several single malts. Vodkas include Grey Goose and Belvedere. Gins include Bombay Sapphire. There is no reason to pay one extra cent for alcohol.
3. Seabourn does offer shuttles in ports. Prices of excursions are commensurate with what you'd pay on any luxury line.
4. The only things you'll pay extra for are spa treatments, shore excursions and gambling. Everything else is included.
5. I never use a cruise line's air department. Since I only business class it's way more expensive to do so instead of making one's own arrangements.
Hope this helps.