"Cross all The North east of Thailand"!!! :-) You are a joke! Google Maps says IT IS 88 km. You don't Have to drive from Mae Sai to Chiang Rai! There is a direct way. I Have Been driving that Many Times..
I've Been flying CR - Bkk at least 25-30 Times. Never last minutes cancellation or Even much late. So Many daily flights so don't Have to worry about all fully booked.