Most of what you are saying is correct but...a new phenomenom related to the ITCZ is coming in to play, and it's not very comon. The sudden increase of temperature due to convective currents (and some sources are saying from -49°C to -19°C at FL 370)... now do the numbers and where that leaves your coffin corner?? suddenly you are flying outside the envelope! I remember a B747 in that same route lost 4000' a few years ago (2001 if I'm correct), and that temperature change range was a factor. Now remember it's not official, but this rumour is begining to make sense, the problem is that it ocurrs very rarely. Temperature is a key factor on your "coffin corner".