Isn't this a false dichotomy? The answers is all of the above, with the varroa and the removal of stores as the two key factors. The population had a shortened lifespan due to varroa infestation in the cells, thus the population got smaller, and weaker, than it would otherwise have been, so couldn't travel to the food you provided, or keep itself warm enough.
A little bit like the analogy of a plane which runs out of fuel at 30,000 feet. What killed the passengers? The fuel running out or the plane hitting the ground? Not a perfect analogy in this situation I know.