@aart , thank you for posting the thread to your experience. It's given me even more to think about.
Maine winters, at least on the coast, typically start off fairly mild, mostly NW winds (which we're quite sheltered from right here) but we're lucky to have even a dusting of snow for the year end/new year Holidays. We typically have a January thaw but February and March is when winter really hits us. If I have my time frame correct, say I set eggs in a week, they'd be 10-11 weeks going into February when our weather is most severe. Weather predictions for New England this winter aren't great (at all) but the worst of it won't really set in until then...so they say. That age at that time seems manageable to me.
I also see that if this hen is broody and I choose to keep her isolated within the coop, I have a few adjustments to make to accomodate such a situation.
I found your observations with your broody rejoining the flock with her babies to be very interesting and reiterates my gut feelings on the two bullies I have here. They waste no time instigating fights and being the last chicks who we raised here, I have no idea how they would respond to babies but I think it would not be good. I've been waiting (for Ichabod's sake and our egg supply) to cull them until the new pullets are ready to join the main flock and I will most definitely be doing so before any chicks are introduced, no matter who hatches and broods them or when.