I dove Similans and Hin Daeng/ Hin Muang in November 2007. I had a GREAT time.
You seem to have the right idea about the various areas.
Drew has a point about it being crowded at some dive sites in the Similans -- you have to adjust your expectations. It can be a zoo at some dive sites (cough-Elephant Rock-cough), almost to the point of skipping the dive; others, you'll be the only boat. Just depends.
As far as photography goes, the Similans were really good for pretty coral (mix of soft and hard) and fish. Pretty blue water, tons of fish. Big fish, little fish, all kinds of fish. Way healthier reef than I expected, based on the number of visitors. We had good to great viz. And both times our boat went to Hin Daeng/Hin Muang (I did 3 back to back charters), we had mantas. One of those times was the best manta encounter ever, by alot (everybody on the boat was blown away). And the similans had lots of creatures like (very large) paired cuttlefish. Good place for a medium length zoom lense.
As far as macro stuff, I didn't see as many small critters as I would have hoped; granted, I didn't have a spotter, due to the way our trip was organized. Saw some Mantis Shrimp, anemone shrimp, things like that, but not tons. I hear there are pigmy seahorses there now though, and some frog fish. And the sea snakes are fairly common and really fun.
If I were you, I'd do a 6 day Similans cruise (the 6 day cruises go to Richilieu Rock, while still allowing a good amount of time in the Similans), followed by a 2 day HD/HM extension. (Our boat seemed to do a pattern - 6 day similans, 2 day HD/HM, 4 day similans, 3 day Phi Phi/HD/HM, repeat.)
I went to Mergui, and loved it, but that was way back in 2005. Lots of green water full of nutrients, and consequently, lots of life. Not sure what the situation is like now though.
The one thing you have to hand it to the Thais for is the level of service on the boats. And they make it so relaxing and fun-yet-safe. I loved the entire experience.
PM me if you want recommendation for boats.
Oh, and yeah, I didn't like Phuket much either. . . stay in Kata or Karon beach for your overnights.
Susan