That airfoil is fairly sharp up front. Have a look at the clark Y airfoil to get an idea of what a gentle stalling wing looks like. I'm not much of an aerodynamics guy, but from practical experience that airfoil looks like it needs to go pretty fast to be efficient. You might consider switching to something that performs well at slower speeds given the parameters of your design.

All a tip stall is is when the tip of the wing stalls but the root does not. Some planforms are more prone to it than others, but all wings can do it. A tip stall results in the quick dropping of one wingtip (like in your video) and no response from the ailerons. Your pilot kept trying to correct the weird roll he was seeing but the plane either didn't respond at all or actually got worse, right? That's normal behavior for a tip stall.
Your tip stalls are happening because of a combination of conditions. First, you have too much wing loading to begin with, making the wing fly at a high angle of attack. So the wing is already close to its stall angle just in straight flight. Second, your pilot is doing those bank and yank turns which increases the wing loading even more and then encourages the wing to sideslip, which makes the inboard wing tip stall. The result is the beginning of a spin, which your pilot apparently doesn't know how to recover from.

For future use, the right way to recover from any stall is to point the nose toward the ground and gun the throttle. This is counter-intuitive since the plane is losing altitude, but it has to be done if there is to be any chance of saving the plane. Overpowered planes can just throttle out of a stall, but you don't have an overpowered plane. Tip stalls require a second bit of maneuvering and that is to rudder the plane in the opposite direction of the unwanted roll. DO NOT TRY TO ROLL OUT OF A TIP STALL OR A SPIN WITH THE AILERONS! It will just make the stall stronger and take away any chance of recovery as happened in the death spiral I saw at the end of your video.