there are no annual limits to land crossings anymore, since about two months ago. When I flew back from Kuala Lumpur last month, I had a gut feeling I was going to be turned back, were that to be the case, my plan was to head north and cross visa land.
In September I flew back from Malaysia after a couple days there and was fine, the following week my friend did the same return trip and was stopped and questioned. He was let in, but it shows a change in attitude, Chiang Mai has always been regarded as an easy airport to enter the country via.
My suggestion would be: take the bus up to the border and cross in to Thailand via land, then travel to Chiang Mai.