For cherry and most fruit trees 40F is optimum for chilling. Below 32F no chilling. Anything above 40F and the plant will start growing after it satisfies it's chilling requirement.

To really keep a plant dormant but not frozen 32-35F would be ideal.

It is very difficult to keep woody stock in greenhouses to maintain dormancy. Overwintering fruit trees is quite different than just maintaining dormancy. I had a 'perennial' greenhouse when I was in business built primarily to overwinter woody shrubs, trees and herbaceous perennials. The big issue if you really want them physiologically dormant is actually maintaining that dormant state. You actually need to be worried more about too much heat than not enough. When the temps rise in a greenhouse (sunny day, transparent covering) they try to go out of dormancy. Some sunny days mid-winter, it can be minus fifteen and yet eighty inside a greenhouse with the heaters not cycling. We put our overwintering stock in an unheated greenhouse, covered it with insulation blankets, and then covered the greenhouse with a winter film (cloudy, opaque and white to reflect sun). It's a fallacy that this stock never needs watered when it's dormant. Not very often, but occasionally we had to pull back covers and water the stock in. That greenhouse was torn down when I retired from the business, and the trees I have overwintered since have been in my personal greenhouse, and I do have a stat I set at 36. It doesn't get below that at night when the heaters kick in, and I do have to provide passive venting on sunny days. I grow mostly fruit trees for my own use, so they get put outside in the warmer weather in pots and by the time they've spent their second winter in my g'house they are ready to go outside. I am not growing them in the g'house for fruiting, just to to get them to survival size to withstand a harsh winter. These trees, although they may shed leaves are not truely dormant and opening windows does not keep the facility cold enough to maintain dormant state.

I'd say that if everything is deciduous (therefore, no light needs), I'd put something over it that reflects light and is opaque. Not black, because it might absorb heat and transfer it inside the greenhouse. White but thick enough to block out light would be best. Maybe vinyl?

Set the heat at 32, and have a ventilation system kick on if it gets over about 40ish.

Temps should never get below 32, and probably only above 40 if it's actually that warm outside.