Ross, look at the total volume Shaf would have managed after session 12: That's 1204 chinups in twelve sessions and if we roughly estimate the average load of the reps (guesswork) at 100kg that is over 120 tons. In twelve sessions. That is probably more pullup volume than Shaf did in the last year(s). Compare that to a GVT approach, reps and volume. I know of no other rep/set scheme which allows to accumulate comparable numbers so easily because every set is easy and no rep close to failure if you choose appropriate loadings.
So I would say he would probably do just fine in session 13 and 14 but if its to steep, use a miniband during these sessions. The loading in session 15,16 and 17 is just guesswork. I would probably have him test his 3 rep max (three chinups with 5kgk, rest 3 minutes, three chinups with 10kg etc) and use that for session 16 and adjust session 15 and 17 accordingly.
I would agree with you observation about lock offs but I think the critical point in the chinup is the transition around 90 degree. If you ask somebody who has only a 10 rep max to do chinups from the top to 90 degree, up and down he will usually knock off a lot more chins... I would set up the bands in such a way that they would offer only little support on the top.
Pavel I don't know but would guess that there are better templates available if you can already do 20 reps...