Have fun! Mae Hong Son is gonna be beautiful - don't miss that when you're up there! Don't worry so much about the weather - Doi Inthanon is gonna be cool no matter what period of the year.
Incidentally, I'm thinking of heading towards Chiang Mai with a possible quick loop around MHS, and then head into Laos sometime perhaps in mid-Apr18. Gonna make use of the weekends and the Labour Day PH to maximise the trip while minimising leave application.