So this is what I miss when I'm away from the forum...man am I missing out! Quality job, I really like the design! I actually wanted to do a similar build, but nothing came to fruition. Glad to see someone pulled it off! Nice job!
I am going to agree with the barrel length being a large factor. Yes, improve the seal since it is horrible, but also fix the barrel. If the darts begin to spin (as you mentioned), it is usually caused by excess air coming out the end that nudges the rear of the dart into an unstable flight path. This effect can be observed when firing streamlines through a snap with a normal cpvc barrel and then with a ported barrel. The ported barrel allows air to exit the barrel slightly before the dart exits to prevent the issue of excess gas. Long story short, either port your barrel or simply make it longer.
I read through your data and experiment (yes, the spanish too), and the set up looks great. I may experiment with the ballistic pendulum, as I have a book with details on building your own. Just out of curiosity, do you plan on addressing the chamber volume at all?