In general, a layer of snow falling and settling on a roof for long periods should be no problem, because it \'breathes\' relatively well. However, if the snow thaws and then rapidly freezes over the entire roof for an extended time, it could as you say form a solid which would have a muffling effect that stops airflow.
By fitting lapVent units you will introduce the ventilation that is needed to combat condensation in the first place. However, there may be periods when they might not be as effective as they should otherwise be; but nothing, apart from lengthy shielded vent stacks will breathe through thick impervious surfaces on a roof.
A sealed roof is something of a debatable issue, any living space produces moisture, this moisture turns into a vapour which rises until it can go no further condensing whenever given the opportunity. A roof is usually the last barrier and the only answer is positive ventilation.
Your position sounds challenging, it would be worth a simple trial, I suggest you contact 07894 459 288 and explain the problem.