Given the choppy nature of the editing of This Is It, I doubt there is a lot more usable footage in the vault. I think they used the best performances they had to hand
I do agree with this to a degree but it's also important to remember they had to stick to a 2 1/2 hr runtime for the film, they cut whole verses out of songs (Beat it) and trimmed down songs to fit in the interviews and the behind the scenes footage, there's definitely more footage, but the degree to which its up to MJ's usual performance standards is the mystery. If I recall correctly here's what I think could be added in a 2-part doc, realistically if they put it ons streaming:
Will you be there - (it was ran twice I believe in the forum)
Heal the world - (was also ran twice, and they *may have* ran the number on the 24th but i don't think MJ was on stage
We are the world - (was rehearsed on the 23rd or some day at the staples center so it could be included)
Dirty Diana - (was ran twice with MJ but from what I heard MJ barely sang outside of a falsetto for most of the runs of that number)
IJCSLY - was rehearsed on the 23rd, and the 19th(?) so they totally could mix those dates in
all the new videos could be shown in full
MITM - the full thing could be shown this time around, I don't think it was run any other time with MJ on stage
SC - the lean was rehearsed on the 23rd, they could show that
BI - the second verse was cut from the film, it could be used
TWYMMF - we know he sang it live in those forum rehearsals and didn't sound terrible, so those vocals could be used
Earth Song - dub with the actual studio "what about us" vocals instead of a demo if he wasn't singing out
Not sure about:
You are not Alone rehearsal - I think we only have one day for this so I can't speak to this one
Stranger in moscow rehearsal - same here, we have those 2-second clips but outside of that not much else
DSTYGE/RWY - (from the same day that Beat it/IJCSLY and Thriller is from, and I think the 23rd)
Dangerous - (has to be a mix of footage bc apparently he wasn't keeping up with the dancers for most of the runs of this number aside from the 24th where I heard he was more on top of it by then)
They also have footage of those early rehearsals in the soundstage, I heard MJ actually showed up to those early rehearsals a lot more, so there might be more there.