Actually the wind was pretty stable on the Mittellegi Ridge on Eiger during the morning, which made is reasonable to fly. And on the summit too. On Mönch the wind was way too heavy and I would definitely have lost it. I am uncertain about the wind speeds, though.
I brought four batteries and charged them, when we were in the mountain huts. When climbing (and not in use ) they were in a thick wool hiking sock to keep them warm. That worked great. The temperature on the Eiger summit was -3 degrees Celsius and the Spark had absolutely no issues taking off or flying