Keeping balloons (or just one balloon) under control on the ground
Grab your mallets and hammer a nice long metal stake or three into the floor. Use them to keep your assembly attached to terra firma without any danger of SPB team members going on an unexpected stratospheric joy-ride. There, now you can use a triple-triplet config!
I also don't think some people here appreciate just how huge a latex balloon can get at altitude. I'm pretty sure a lot of people know about the infamous expanding capabilities of condoms though? Well, this is the same stuff on a vastly larger scale. Wikipedia quotes a 100:1 expansion ratio. Think about a balloon so large that you could stick a couple of Jumbo Jets inside of it, and you're getting to the sort of size a latex weather balloon will expand to before bursting.
Dunno how Lester thinks that three balloons will only go as high as one. You would only have to inflate each balloon a third as much, and that's not including any other triplets also attached to the launch platform. I know my idea was a triple-triplet set-up where each triplet can lift the entire platform on its own, for a little added redundancy. Inflate the upper set the least.
Anyway, as for deciding when to launch, there are a number of ways. One is the obvious pressure switch. Use a solid state barometer and have it used to initiate launch sequence when air pressure readings drop below X, indicating you are Y feet above sea level. This could be backed up by some system to detect a sudden altitude drop of say, more than 1000 feet. Just in case a single-balloon set-up bursts early. The good thing about going solid state, is that it has no moving parts to freeze up and screw with the release like with PARIS.
Another redundant system, which could also provide some other benefits: I know the Reg Guys were thinking of (but eventually never bothered with) tracking via telescope for PARIS. What would be the chances of not only tracking optically with a telescope, but pointing a radio dish in the same direction and keeping a live communications link with the thing? Everyone gets to see the telescopic view, plus with a singular balloon platform you can see yourself if your only lifting device bursts and can hit the big red button for a manual launch from ground control. Maybe creatively re-engineer a couple of TV satellite dishes for the purpose? What's the regulations in Spain with regards pointing a few watts of radio energy into the sky?
Oh, I know I keep saying this, but CAMERAS EVERYWHERE. Two on the Vulture 2 for forward/rearward footage, preferably with the rearward camera able to see any rocket-pod-jettison action if indeed there be any to see. At least two on the launch platform. One pointing downward toward the Vulture 2, the other upward toward the balloons to capture the burst and parachute deployment. Maybe an extra one pointing sideways towards the expected direction of travel for the little spaceplane.
I keep making long posts don't I?