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.
It's completely up to immigration. I entered Thailand via Chiang Mai Airport from Malaysia on Saturday, and immigration didn't bat an eyelid....my friend made the same journey today, him and others were pulled aside and grilled by immigration!