Cold temps, frozen release mechanisms, and reducing weright
Here's my $0.02 on barometric release mechanisms. Keep it simple, and think about the COLD. I was about to suggest an electronic solution based on a chip pressure sensor (the electronics weigh little if you use tiny surface-mount chips and avoid heavy packaging for the circuitry), but batteries tend to freeze at -40 C. Still, it may be more workable than a mechanical system. You may need to use heaters (probably left behind on the balloon at release) to keep the electronics functional.
For a mechanical release system, consider the flat, disk-shaped bellows used in an aneroid barometer (but filled with air so it expands at altitude to trip a release lever); simple, can be calibrated to operate at the right pressure/temperature, should be very reliable.
Speaking of very cold temperatures, that's going to be THE major issue at altitude; making the systems (primarily the batteries) function at those cold temps. By the way, the cold will raise hob with most mechanical release mechanisms like the syringe-plunger actuator.
Hypobaric testing is of minor importance compared to low-temperature testing that mimics the temperature profile of the intended ascent.
Here's another thought on electronics, light weight, and thermal issues: Sandwich the electronics between flat sheets of thin foam plastic that form the fuselage/wings; you can get flat batteries that can be insulated this way, too. For structural strength, you cannot go wrong with a strip of carbon-fiber tape as a wing spar. This is frequently used for RC model sailplanes; glue the tape to the top and bottom of a floppy foam wing, it becomes MUCH stiffer.
Another thought on weight: If using off-the-shelf electronics, strip off all the plastic cases from everything to pare down the weight.