I can say that in my more than 20-years of observing hardwoods that I can probably count close to four times I thought I saw bud swell sometime in late winter (inside a heated greenhouse for dormancy storage of hardwoods) but only to see I had imagined it. I was paying what I thought at the time closer observance to bud scale separation all to find out later I had imagined it all. Like you, it all came about when I convinced myself that the buds has swelled noticeably…
Should they leaf out it’s no big deal, but know that if grafted scions ever break for any reason they need a lot of light once you see the leaves unfurling and if you cannot provide strong light for them, they will likely crash and burn.
To answer your concerns, should buds actually swell, scales split, and the unfurling of leaves - you can wait more than a month w/o hindering the success rate of grafts.
For your information also, this greenhouse of mine will be 60-80 degrees in there every day the sun is bright and shining from the moment I close the doors in Fall till the day I re-open, come Spring. An exhaust fan and motor-controlled shutters runn full speed during a typical late January day at 10-degrees F with the sun shining and the greenhouse at 70-degrees inside. This goes on every day the sun shines thru winter. There are times when hardwood’s wake up during winter, there’s no doubt about that, but it’s always at least mid-Feb. here in IL zone 5b that they do.
It’s gotta be on the same lines as the relationship among chill hours for folks living in areas of the world where a certain # of chill hours must be met in order for that hardwood shrub or tree to wake up thus breaking dormancy. My greenhouse is set for 2-degrees above freezing. I simply do not wish for the roots to ever freeze.