hoistop, thank you for your insights. It's been several months since I originally posted this and I haven't been near to real mountains since then
I am aware of 2 independent flight crews among my aquintances trying to fly to the same area since then one had to chose an alternate upfront, the other had to turn back after some heart-pumping manouvering in a valley and an unsuccessful attempt to fly over the top of the clouds instead. So it's actually not an easy path and everyone is happy to be back home safely. Much of the advice collected from this thread is actually visible in the video description, hopefully serving pilots that aspire going the same route to those mountains.
I, on the other hand, chose to fly some flat-land mission in the autumn (let's leave the mountains for the pros!
Ironically, it seems no matter what I do, I end up in some trouble. In my case it was fog and low-level clouds, making me fly on-the-top VFR for about 30 minutes. I didn't dare to put the link to that footage up here to avoid getting flamed. So the luck bag is getting thinner and thinner, mountains or not.
Anyways, take care and thanks for advice on mountain flying.