I did Mont Blanc last August, and had quite good conditions.
If you do the Gouter route, you'll have a fairly long walk in (although not as long as mine since the Nid d'Aigle line was shut when I was there), so the K4S might not be your best bet there as they are pretty flat-soled. If you are doing the Trois Monts route then you'll be on snow pretty much the entire time (unless you do the Trois Traverse), so it wouldn't matter as much.
I wore Scarpa Mont Blancs up, and my feet were blocks of ice. Well past the point of being able to feel them or even tell if they were wiggling. Hands were very cold, too, wearing ski-weight gloves.
That said, once I got to the top and stopped, pulled out my puffy, and started back down, everything warmed up just fine. I was only wearing a thin shirt, fleece, and thin softshell. If I had been willing to stop earlier and put on a warmer layer on my torso I'll bet I would have been better off with regards to my hands and feet.
If I were to do it again, this is what I'd do:
Purchase a set of these small neoprene overbooties (ok, I already have these).http://www.backpacker.com/gear-zone-gear-review-crescent-moon-over-shoe-booties/gear/15159
Wear your Trangos for the weight and walkability, then after you pass the Grand Couloir and put on your crampons, throw the neoprene booties on, as well. They will work fine with strap crampons, and might take a bit of trimming for hybrid attachment. Or stick with the lighter boots and wear thicker long underwear.
Bring both jackets with you to Chamonix, and decide the night before based on forecast. I could have gotten away with a lighter one (like the SD Gnar, instead of the Brooks Range Mojave I brought), but a few days plus or minus my summit had some very cold, windy temps.
You'll see long lines of Asian groups wearing, no-kidding, Himalayan suits.