Thermostat advice?

I would recommend against a setback thermostat to boost daytime temperatures. Heaters should be used to maintain a minimum temperature, and let the sun do the daytime warming.

If you use a heater to artificially increase the daytime temperature, that increases the plants' metabolic rates, but if the solar flux doesn't happen to cooperate (i.e, it's cloudy), you end up with weak, leggy plants.

 

You can do this by installing a heating thermostat in series with a cooling thermostat. The cooling thermostat closes when a rising temperature reaches the set point, and the heating thermostat opens when the rising temperature reaches the set point. The circuit is closed (and thus the vents open) only between the lower cooling set point and the higher heating set point. So you might want to set the cooling stat to 80f and the heating stat to 90f (or whatever). I really think you might be straining at gnats to do this, though. You probably will find that leaving the vents open while the cooler is running works just fine. Think about where you locate the thermostats. They should be shaded from direct sunlight, but still in the flow of air, and at the elevation of your most sensitive plants.
I don't know how your wiring is planned, but I suggest you group as many things on separate circuit breakers as possible. Separate the cooler from the vents from the outlets, etc. If you are thinking about work lights (and you should be), install enough so you can actually see enough detail to pot, etc. - at least three 48" two lamp fluorescents (in moisture proof enclosures, if you can afford it) with 5000K lamps (color temp.).
What have you planned for internal air circulation?
I didn't say it in so many words, but locate all your cooling thermostats in one location.

 

Marni gets more sunny days in winter than I do and like she said, where I am in east SF Bay, winter weather can be dark/cloudy/rainy for sometimes weeks at a time, and the temperature inside my greenhouse may not rise above 55°F without the help of a setback thermostat. I am glad I have a setback thermostat. I don't know Oklahoma weather well, but if you have decent sun during most of winter, your greenhouse should warm itself sufficiently in the daytime. Then again, there is a lot to be said for being prepared for as many possibilities as possible. I am glad T. migratoris is encouraging you to have a set-up where it would be possible to add a generator in the future. Loss of power when one is at work, out of town or otherwise indisposed, can be potentially devastating. A local grower I know lost nearly her entire collection when the circuit breaker flipped and she was out of town, leaving the greenhouse to overheat and cook all the plants inside.

Growers in your state or neighbouring states I'm sure have better advice for what it takes to cool a greenhouse in your climate, but I think it's good to have roof vents just in case, even if you don't use them. I am glad msaar is giving you lots of advice being from a nearby climate (that I assume is similar?).

I have two small greenhouses outdoors. One greenhouse is kept below 80°F and it has no roof vents in use (there is one for emergencies) and is cooled by a wet pad on one end that comes on at 75°F, intake shutters at the wet pad open at the same time as the exhaust fan at the other end of the greenhouse at 80°F (different thermostat). The suction method ensures that all the hot air is evacuated and than any new air coming in passes through the evaporative cooling pad. On summer and early autumn days even with all new air passing through the evaporative cooling pad, humidity inside the greenhouse can drop as low as 55% if it weren't for the Hydrofogger than makes up for the difference, set using a hydrostat to 80%. Because the cooling is active (no passive cooling through roof vents) it is essentially a sealed greenhouse except for when the exhaust/cooling combo is running. Being "sealed" like this makes it possible for the Hydrofogger to maintain the humidity at 80% throughout the greenhouse without any drier spots.

The other greenhouse is cooled only via passive cooling, with a large roof vent. It also has a hydrofogger inside, but it has to work a lot harder to keep up the humidity when the roof vent is open, and even then the humidity levels are uneven. I can only get by with this method of simply passive cooling because of the mild weather here. Ideally I would change it into a system like I have in the cool house, but it's not feasible due to various limitations. Many orchids do well inside, but only in certain spots that farther away from the roof vent. Also the ceiling is far too low, so this is a factor.

A few greenhouses here in the Bay Area of excellent growers use a combination of passive cooling though solar vent openers of vents along the apex of the roof, coupled with evaporative coolers (not wet walls, since they only work when there are no open vents). Water is added to the floor by misters or by hand several times a day, especially during warmer, sunnier days. This is for climates with day time highs of 75-85°F on average. I think you probably have much hotter days to contend with. T. migratoris has much hotter days in the Sierra Foothills, and so does Reyna and others. Hopefully you can use their advice to decide on how to cover the bases for cooling.

My advice about how to address the doubled-up thermostat issue has already been explained very well by msaar. The one you showed me from the FarmTek catalogue won't work for what you want. I think it may be difficult for your contractor to fully visualise what is needed in terms of cooling.