Megbun,
Sounds like you've done your homework, and are in the problem solving stage all dahlia growers have to work through.

As to your first question, most growers lay the tuber on its side, with the sprout facing up. The roots, as long as you cover them, will grow towards moisture. Without a picture, its hard to tell how I'd trim an oddly shaped tuber clump.

With your second question, I had flats of well-sprouted tubers outside in what I thought was 45-50 degree weather in order to add potting mix to the containers and water thoughouly,and had some cold damage on several plant leaves after I came out after dinner to bring them back inside. I was surprised, and a bit chagrined. Perhaps someone else can quote you exact temps...

You might consider a cold frame, or a vertical 'greenhouse' that is basically plastic over a plant stand. That might add a bit of protection when the temp dips more then you reckoned on.

And to answer the question in the headline - they need light when they break the surface - but not if you are going to continue burying them for a while.

Lack of light will make for a long, weak spindly stem.

Gotcha (RE: sunlight). I also got a bit of an answer this morning from one of my tubers. One of my sprouts sticking out of the dirt actually has a couple of tiny little leaves on it! Very exciting. I assumed I was just going to kill all my tubers (not that I'm out of the woods yet).

So my question now: before developing leaves, do the sprouts need sunlight? Is leaf growth triggered by light hitting the sprout? Or is it temp? Level of root development? Something else?