My heart sank when I saw daylight coming in around the chimney! I know that feeling all too well after working hard on a project and something just has to come along and spoil the mood.
Ice and snow on roofs can do all sorts of goofy things. I remember dealing with a variety of problems over the years when my dad was a builder up in CT. Sometimes the solution is opposite of intuition, especially when issues are caused by some combination of slope, exposure, heat, and insulation.
Putting a snow splitter uphill of the chimney (and any vent pipes, etc) will take care of general problems due to gravity loads. The snow splitter does not need to be fancy, it just needs to divert snow around the chimney when it begins to slide. The simplest ones look like shark fins with some wings.
I assume there will there be a thimble/trim/plate/etc installed on the ceiling once everything is finished? That will be important to eliminate freeze/thaw issues around the chimney collar that can be as bad or worse than snow/gravity loads. Any sort of temperature gradient or insulation difference or abrupt temperature change reaching the roof will exacerbate snow/ice problems significantly. It could have possibly been a contributor to the current problem.