I am in agreement regarding cutting down - less effort and more gravity/natural cut. I am not yet sure about cutting up - what is most effective (gravity doesn't work quite so well!).
I noticed that by the 3rd kesa cut, you seemed to be concentrating on relaxing more and you extended less by definition.
Regarding the upwards cut, it is not my strong point either. Centripetal force is still necessary. Munenori say to keep your rear hand no more than two fists distance from your hara. That has been a good start for me.
I would love to hear how things progress. I am also in the perpetual student phase on cutting..... at least until I can learn to cut the silk cloth in mid air.
But even then, I will probably figure out that cutting the silk cloth is just the beginning of the next pathway.