I haven't read the whole thread so i'm not sure if this has been discussed before,but anyways..
I think the reason he sounds soo different from previous tours other than the fact of him beein sick at the time and age is this:
The songs on history tour (that are playback) are all on the same tempo and key or even slower than tempo as the oroginal records. This is very important to notoce. What does this mean performance wise? MJ has to actually pronounce all the lyrics and words 100% when he sings them. Just like the records. No live artist ever does this. Thats why you have those subtle changes in arrangement and words being swalloed and left out, if that makes sense. For intance on Dangerous (and Bad and Victory) tour during BJ he would sing the song in a much faster tempo wich allowed attistic freedom upon delivery of the lyrics. The live lyrics often is half pronounced like "Peepa aways toad mee be cafa what u doo" Where as in History he has to actually sing the lyrics in the SAME tempo and pronounce the words 100% correctly. "People always told me, be careful what you do" to match the playback. IF he sang it with the same artistic freedom like dangerous tour it wouldnt match the playback.
Also I think MJ much rather sing under playback than mime the lyrics because of the feeling and emotion of him belting out actual voice. Giving a much more realistic performance for the Audien e.
For me personally when I put those two things together I can understand why he sound the way he does. Im sure if he chose to sing live during history tour with songs in a faster tempo and sing the lyrics the way he used to, it would habe sounded much better. Not like previous tours but better than under the playback. But than again he would have to dance faster, wich maybe explaines the reason why he chose playback.