Yay, first reply. I know the replies below are going to say "It's too busy" or "it's too rushed". Yes it is possible, and yes it is doable. However it is really busy.
The first thing that pops out to me is, how do you plan to get from Kuala Lumpur to Ko Samui? There's no direct flight. At least with Phuket there is a direct flight.
I did 48 hours in Siem Reap. Arrived around 3pm, sunset at the temples, full day of temples, then morning at temples and finally fly out. It was enough time to see the temples, but I really want to go back just to see more of Siem Reap.
Not enough time for Phuket/Samui. Not to mention getting to Samui would take even more time.
How to make it better? Either eliminate Kuala Lumpur or Phuket/Samui. Spend one extra day in Bangkok and one extra day in whichever one you didn't eliminate.
There's nothing quiet and relaxing getting to the beach, spending 8 hours there, and then leaving the next day.
I can't help you on the points/hotels because you and I are completely different on that part.
Anyways, have fun!! There should be some good replies coming up soon from other members.