Q - imagine if you got the gopro tangled in the lines
A - This is possible if you position the camera in front of you. But if it is behind you or even on the sides chances for that are very slim. During our testing we crashed many times to the water and there was no issue with that. But I cannot say the risk is zero.
These videos are nice to watch. But, if you start selling this type of mounting systems, sooner or later one of your customers will
surely get a kiteline tangled around the mount. As you say, the risk isn't zero, so given enough customers... You know that some people are going to try to do rolls-to-blind or handlepasses while filming themselves... Other people will crash and get rolled in waves. So, since line entanglements will surely happen, I think you ought to test this beforehand to see what will happen at that point. Will the mount safely break away? If not, will it be easy to free yourself from the backpack?
Getting dragged backwards through the water with a death-looping kite entangled in your backpack system sounds like it could end badly. It wouldn't exactly be good PR for you or for the sport either. So, if I were you I would do some testing upfront.