@PoptartProdigy, can we perform the Final Flash? I think we can do it and let me explain why.
Vegeta, when he went to fight Cell, was utterly assured of his victory. He thought there was no possible way he could lose when going into the battle against him. He did not know how good his regeneration was and he didn't know about his Perfect Form until the end of the battle between him and Semi-Perfect Cell.
With such an extreme - and well-deserved - confidence in his superiority over Cell, plus the fact that he didn't know he had to destroy all of Cell's cells, I don't think Vegeta invented the Final Flash in the Hyperbolic Time Chamber. He simply wouldn't have thought he'd need such a technique. Instead, I believe that Final Flash was something he made up on the spot. It certainly goes into explaining the long charge time versus later uses of the technique.
With that in mind, we would not be coming up with the technique from scratch. We know about it, we've seen it in use up close and felt it with our Exceptional Ki Sense, and we can probably wiggle out a few sentences from someone for a very basic explanation of how to do it. I reckon Kakara should be able to do it, even if her proficiency with it would be so low as to require the long charging time that the first Final Flash required.
I have another question, one unrelated to the above. With Exceptional Ki Sense, can we sense how much ki someone has left in the tank? Goku seemed to be able to do it, sensing the practically limitless quantity of ki within Buu.