I've got 2 of those little plastic greenhouses that I mostly just use inside for starting seeds and then outside to harden plants off. But I've also got cattle panel arches. The last 2 years I moved plants outside into the greenhouses in the start of May (can plant in ground after Mother's Day but usually wait until closer to the end of May). I'm wondering if I wrapped the arches in greenhouse plastic and put the mini greenhouses inside the covered arches if it would be just enough warmer to move them outside mid April? I learned last year that the mini greenhouse wouldn't keep temps above freezing since I used one outside mid April to start some cool weather crops and there were a few nights it dropped down below freezing by a little bit. Thoughts?

You would probably need a heat source of some sort. I have built makeshift greenhouses for plants and used old kerosene lanterns as a heat source. You can also put candles in cans - the metal heats up and makes a nice warm spot. If you have electricity, a brooder lamp works.

PS: Remember, heat rises so you want you heat at or below the level of your plants.

I think it's worth a try - I've used low tunnels inside my high-tunnel GH to protect plants early in the season. According to Eliot Coleman ("Four Season Gardening"), each additional layer of protection takes things up one "zone" (or is it 1/2 of a zone? can't remember for sure). And if you can add a little additional heat as per Daisy's suggestion, that certainly couldn't hurt either! Even just putting some jugs filled with water near the plants can help - they will warm up during the day and give off a little heat overnight, or at least moderate the temperature in their vicinity.

I don't think I would trust your weather (or mine!) in springtime. This year, the second half of April was beautiful, so I planted tomatoes and cucumbers in the unheated greenhouse, a solid building attached to the house, with plastic sheet over the front end. First two weeks of May, winter came back and took out half my young plants. A mini-tent - plastic sheet hastily stapled over the bins - helped, but I had to lug all the peppers and some of the tomatoes back into the heated part.
Bottom line: It's a worth a shot, but make sure you hold back spare seedlings, just in case.

The last several years weather has been fine where I'm at in May (never dropped below freezing and may have actually always been above 40 degrees F). Plus all my garden stuff is on the south souder of the house so it gets more sun and less chilly wind. It's just those few nights in the 2nd half of April I'd need to protect plants, and even then it's usually only below freezing for a few hours. I'll probably have tons of extra plants to sell anyway so might give it a shot. Someday I hope to have enough land to justify a permanent greenhouse with heating but for now I make due with what I've got.

Just take care. The climate, she is a'changin!