"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.

Assuming 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.