This past spring I read a lot of forum comments about the danger of cracking the winter propolis seal too early, resulting in chilled brood and cluster.
Red Maples were blooming January 18,
However, we still have temperatures of 29-36 degrees at night in February.
So I waited to March 1 for my first inspection and found sealed QC. March 8 they swarmed.
I estimate that the queen layed the QC egg by February 21. (HoneyPump, I lost the link to the brood rearing chart you shared.)
I estimate they were already thinking of swarming two weeks before that? (February 7)
So...
Do I crack open the hive at the end of January this year?
Knowing that night time temps will drop to near freezing, and day time temps only 45-57 degrees, do I still begin inserting empty frames into/around the brood nest of my long langstroth hives?
I would remove the lid and take a quick look anytime it is above 55 F., sunny, and calm. I would pull frames for 30 seconds each any time it is 60 F. and same conditions if it will remain that warm for 2 hours or more. They will reseal the propolis in 2 hours if it is not intentionally removed during inspection.
They did not swarm without drones. So if you see drones then you are pretty safe to open a hive which is weeks before they swarm.