frozen eggs won't hurt things, but usually the shell cracks, which will let in bacteria, when mine freeze, and they occasionally do, I just use them up in a day or two.
I am not surprised that you were surprised, 17 weeks is early to start laying. Sounds like you got more than one! Adding the light might of helped get them to lay early, although my pullets started laying this week too. Put the nesting boxes in, add hay and a round rock or golf ball in each nest.
I do not add any heat to my house, and often the poop is frozen. I would assume that you are colder than us though for longer periods of time. We are quite quite dry, so that helps too.
What I have found out with the cold is that not all cold is the same. Dry cold, wet cold, windy cold and and just **** cold, haha
If you have good protection from the wind, but not air tight so that the humidity builds up, birds breathing, put out a lot of humidity. You are fine without added heat IF you have enough feed. Enough birds help too, more birds mean more heat.... but more humidity too. 29 birds should put off more than enough heat, I don't think you need the light.
Birds that are younger, healthy, and have enough to eat can take **** cold weather and not even seem to notice. A lot of coop fires are caused by added light or heat, which is another consideration.
As for the poop, it really does not stink or decompose or add a health issue as long as it is frozen. When it thaws, your nose will tell you, and you can mix it then or pull out the big clumps.
MrsK