I've done the Trou de Glaz to Guiers Mort through trip in the Dent de Crolles system (in the Chartreuse, next to the Vercors) in midwinter. The caving was fine, but the road up to the Col des Coqs was iced up. We had no snow chains, so had to slog up the hill from the bottom to the top entrance. Hard work in the snow. With snow chains (and of course two cars - one to go up the Col, and to be picked up after the trip, and the other to be left at the bottom) it would have been easier. Other traverses such as Glaz to Grotte Annette would also be possible, I think, though the path from Annette entrance back to the Col des Coqs is steep and exposed in places, so might be awkward in wellies in the snow.
The water in the Gournier would be very cold in the winter. And you do get wet in there. Maybe it would be ok with thick neoprene or a neoprene vest and a wetsuit on top. This cave is somewhat weather dependent anyway, but I do think it gets done in winter.
As for flood pulses - well, watch the weather forecast.