- skip the stopover on Tokyo
- 3 days in Chiang Mai is the minimum ... the more days the better
- summer time at andaman sea is low season, which means that there's rain more often, so you better stay at Koh Samui (there's a direct flight from CNX to USM with Bangkok Airways) or even Hua Hin
- skip the transfer day in Bangkok ... you loose one day just for travelling
- if you want to buy a suit in Bangkok you should stay longer in Bangkok ... to tailor a good suit will take time
- the Plaza Athenée and the Meridien in Chiang Mai often do offer some nice rates, so you better check out for a paid rate there instead of "wasting" points

So my schedule would be like

Day 1: Flight to Bangkok
Day 2: BKK
Day 3: BKK
Day 4: BKK
DaY 5: BKK - Chiang Mai (noon)
Day 6: Chiang Mai
Day 7: Chiang Mai
Day 8: Chiang Mai
Day 9: Chiang Mai to Koh Samui
Day 10: Koh Samui
Day 11: Koh Samui
Day 12: Koh Samui
Day 13: Koh Samui
Day 14: Koh Samui
Day 15: Koh Samui - BKK - USA

Less travel, more time for relax, more time for enjoying Thailand. You could also choose to go directly to Chiang Mai and stay in Bangkok at the end of the trip. So you avoid to carry all the stuff you shop in Bangkok all over Thailand