I wonder how it became common practice to whitewash greenhouses in our climate where light is so precious. I always encourage and make the most of it. All seeds are started and grow in full light and warmth except for lettuce which, in hot sunny weather only, I germinate under the greenhouse bench.
Hardening off is standard and sensible practice, I find a day or two is enough and I do not bring plants in at night. It is often made to sound immensely complicated! Well raised plants in good organic compost are strong and sturdy.