The 'challenge' I see is dealing with a potentially wide temperature range and possibly some humidity. If only it were constantly dry and cold, or wet and medium cold, that would be much easier 
I am thinking a combi of either a down bag and synthetic overbag or two downbags in my army bivvy for warmth and versatility

I am thinking a combi of either a down bag and synthetic overbag or two downbags in my army bivvy for warmth and versatility