Now I'm back to happy.
The greenhouse is still reaching 90-100 degrees every day, and a low of about 60 at night, roughly 25 degrees higher than the ambient temperature.
Everywhere I look there's more watermelons.
The variety of watermelon I planted was sugar baby, from Harris seeds. Germination was good, and as shown, I probably have enough growing season to plant two crops of watermelons a year in my greenhouse. It's interesting, because these watermelons sell for $6-8 each in the local stores, and I think that they would be popular in a farmers market settings. I grew them without pesticides or any treatment at all and they're doing great.