Itinerary looks good. My only concern that has not been addressed are the long haul flights. It takes a day to get there and a day to get back, but you state you have 10-12 days and you list 11 days of activity. I assume you're coming from Boston? You will have several flight routings possible to Chiang Mai, but the Ko Samui part will be different. I don't know what flight times are like out of Samui airport, but if you're flying out of Samui at late night, then you will most likely be spending the night in that city you will be arriving to. You might end up booking multiple air tickets as well (example : Silk Air flight to Singapore, then Singapore to Boston on some other airline).

So perhaps you should do some research on the long hauls, or at least tell us about what you were thinking.