Zymosis

Your video indicates an internal pressure of ~65 kpa which is ~9.5psi. That number sounds off - my inflatable paddleboard is rigid as a board at 9 psi. Do you have any pics of your measurement setup?

You might want to consider a GoPro Hero 4 (I prefer the Silver because of the touch screen). Pretty solid camera, huge aftermarket support for accessories and snag-free mounts.

For the external control/indication, there's a new accessory for GoPro's that you might want to check out:
http://www.poalabs.com

So I ended getting an a6000 with the 16mm prime lens. And picked up an adapter so I can use my tongue switch. Wooo!

Is any way to hook up a remote status indicator like the Hypeye? Has anyone done or heard or something like this? Does the camera even have the proper outputs for something like that?

What is it that makes the Sony cameras so well suited for skydiving, besides form factor? The autofocus? Anything else?

Is there any way to run an external flash with the a5100? If not that might push me towards the a6000 because it has a hot shoe.

I've been considering the Sony A5100, and am curious what other small/light cameras I should take a look at.

Thanks!

For what it's worth...

I work with 3d printing very frequently. There are a number of different technologies out there, each with very different characteristics. From what I've seen, the Grellfab mounts are made on FDM machines (basically, squeezing molten plastic filament through a tube). Which is great for low-res modeling and prototyping, but it's not that tough or durable. Build layers can separate, stress fractures occur easily, and so on. FDM isn't terrible if you have large build features and build with nylon (vs ABS or PLA, which are FAR less tough), but still not something I'd trust a $500 camera with.

Personally, I think the GrellFab design isn't inherently bad, but they should be building their parts with SLS (selective laser sintering, typically nylon). Nylon SLS parts are extremely durable compared to FDM parts, but they require a professional-grade printer instead of a $500 FDM printer that you can build yourself.