Hi

Yes, too early to plant out you have to wait till all the frost have gone and depending where you are in the county it could be sometime in late May early June.
Placing the pots outside during the day will slow them down but I would keep in a sheltered part of the garden as we are still getting cold cutting winds which may scorch the new leaves I would prefer to keep them in the greenhouse with all the windows open if you can ? you can always pinch them out to create a bushy plant.
yesterday here with me I had a hailstorm that lasted 10 minutes , still only March
Is your greenhouse heated ? as mine have only just started to show new growth

Hi, welcome to GoY, you could always take more cuttings, as well as pinching them out, as long as you leave at least 1/8" of stem, they will shoot again, probably with 2 or more stems, and any cuttings taken this year, will flower this year, and form small tubers, Derek.

You can pot up the dahlias and place them in a sunny window inside the house. Keep it watered and let it grow. When the temperatures have warmed up sufficiently, acclimate them gradually each day to the outdoors. They can be transplanted into the ground, or you can just keep them in pots. My tuberous begonias also started to sprout leaves. I have it growing on my kitchen window.