"What do you think? doable or too hectic?"
If it was my trip, I would personally think that it was too hectic, and with only two weeks choose one of Thailand, Malaysia-Singapore, or Indonesia, not all three. But you have different ideas on how to travel, and I respect that.
buttuming you want to visit lots of places, you should look at discount airlines like "Air Asia" and "Bangkok Airways" that have dirt-cheap flights in the area you are considering. If you try to visit several of those places without flying constantly, you would have a hard time doing anything more than travelling from place to place - transport by land and water can be very slow in southeast asia.
BTW, some Thai islands have excellent scuba diving (like Koh Tao), and there are lots of trekking options in Northern Thailand around Chiang Mai and Chiang Rai. Spending 4 days in Chiang Mai including a trek, 4 days in and around Bangkok (you could see the Bridge on the River Kwai or the Ayuthaya ruins), and 4 days on Koh Tao including diving would include most of what you wanted to do, and would be much more manageable travelwise.
