We can dive in the Andaman Sea all year, but the prevailing winds, coming from the west, make swimming at our sandy beaches problematic due to rip currents, so at that time of year we have many fewer visitors. With fewer visitors, dive centers at outlying islands like Koh Lanta simply don't have enough customers to make dive trips feasible, and in addition, boat travel can be problematic with the small boats that dive centers on Koh Lipe employ. From Phuket we use large boats that can handle the sea conditions, and we take divers to the sheltered sides of the islands, so we still dive. What I'm saying is that "wrong time of year" is not accurate for the diving itself, but rather for the passenger loads at the smaller islands. There are liveaboards departing from Phuket that visit these small islands, however.
Not really. Just because there's no airport doesn't make it more rustic. It's every bit as developed for tourism as Phi Phi, in my estimation, and even more so perhaps since there is actually automobile traffic there. All of the islands you mention have areas that are more built up and areas that are more rustic, and this includes Phuket (perhaps especially Phuket given that it's Thailand's largest island and really only a small part of it is developed).
If you want a Gulf island that's less developed than Tao, you might like Koh Pha Ngan, but not near the site of the full moon parties. The rest of the island is more rustic.
