iphone launched into space
I've just re-viewed the youtube video of the guys who launched their iPhone up to 30km. I've also just watched the video of the guy who jumped from a balloon at the same height back in 1960. From the two videos I can say that the ascent through the thermal differential layers of the atmosphere is going to require a lot of serious hardening of any designs for this balloonery. If not taken into consideration, in all likelihood, any loose cables will become seriously tangled.
The other thing you get is that there is NO SIGNIFICANT ATMOSPHERE in which control surfaces could possibly work. The guy who jumped out of the balloon was falling at 1,000 m/s and the air wasn't even ruffling his hair.
It's probably a good idea to use a larger balloon and to leave it slack at the surface on launch. We learn from scuba diving that as a buoyant device ascends, it also expands and/or experiences increasing internal pressure and we don't want our balloon to explode before topping out. Also, some sort of heating device inside the balloon would provide some extra lift if held until the last.