In my experience and knowledge of singing it can be harder to hit extreme ends of your range while sitting than standing. It's harder to sing in general sitting. Especially when playing a piano where you have to be kind of hunched over.
I don't know if anyone else has mentioned that fact; I stopped reading through the whole thread because I realized I don't really want to know that much; he's obviously not doing it the bulk of the show (mess-ups, going slightly off-key, laughing, burping or stopping the song to talk make that obvious), and with music in general I'm kind of of the mindset to let myself keep my head in the sand. I also think a lot of bands playing shows so elaborate and reasonably long use some backing tracks or assistance here and there and it doesn't bother me ... for shows that big to some degree it crosses into theater, and to me that's just a reality. I do think whatever Coldplay is doing, they are trying to make the best show they can ... not doing it out of laziness or trying to rake in money for less effort or anything. Bit odd and sad they do it the whole of Clocks, which is one of my favorite old songs, but when I'm at a show I'm personally singing too loudly and badly to tell anyway so I guess I'll be fine.