Currently it takes only the top three block makers combined to surpass 50% of the network - for at least the past six months it has taken the top four pools to pass 50%. On a positive note, the private to public ratio seems to have been stable for the past few months.
A high price alone correlates with high centralisation. A high hashrate alone correlates with lower centralisation. A high hashrate at the same time as a high price has tended to average centralisation.
Most or all of the missing bitcoins were stolen straight out of the MtGox hot wallet over time, beginning in late 2011. As a result, MtGox operated at fractional reserve for years (knowingly or not), and was practically depleted of bitcoins by 2013.