I am having big problems believing GOX lost 800.000 bitcoins in a couple of years because of the transaction malleability issue.

What if Gox , while it was growing in size, just like the banks decided: "We do not need to hold 100 percent of customer bitcoin in reserve, because at no time will all the customers withdraw all their bitcoin from Gox. "

and then instead of lending out customers bitcoins to others, for interest,  Gox decided to simply sell a percentage of the customer bitcoin they held in reserve huh ?

They could not lend out coins they had in reserve because the public would then immediately realize Mt. Gox was fractional reserve banking bitcoin, but if they just sold them on gox themselves pretending it was customers selling them, nobody could find out.

If Gox did this then they were fractional reserve banking bitcoin, diluting the amount of coins (IOU's from Gox) poeple believe they have, and have been driving down the price of bitcoin.