The Vikings colonized Greenland to re-supply their ships with livestock, grains, vegetables, and everything else they needed to move onto North America.
Whre are you getting such weirdass alternate history from? The vikings couldn't farm for shit in Greenland. A little barley was all they could manage. They had to constantly import food.
So what is the "perfect" temperature and water level we should maintain, who decides those things, and how do we do that?
The temperature that human civilization evolved with.
Why do people think that's a difficult question to answer? We're trying to save the humans. The "meh, let the bad things happen" attitude seems to be very anti-human.