For this kind of mission a twin is essential. At least if you live by the rules asI have set. I will ver fly over the Alps when there are clouds. I once was in bad weather over the pyrenees and that was enough. In case of an engine failure .. You are screwed
A twin is certainly not essential for that mission unless you have personal rules that make it so. There are plenty of singles that fly high and IMHO are fine over the Alps in cloud. It is also a personal decision based on you view of the likelihood of engine failure.
A modern single with radar, de-icing and even maybe synthetic vision I would take in a second over a clapped out Seneca. Stick a PT-6 on the front and there is no argument....