Why the repeated call for "best possible way" ? The "best possible" way to do any one of those steps might take a few years to implement (after several decades of research to identify it as being the best possible), and might only improve the fit by (say) 0.2% over a lesser method.
Your steps make no attempt to identify the content of each slice, and no attempt to identify the density of each portion of the volume, so you are only going to be using some kind of statistical mean of human body density when you estimate the mass. Don't you think that might introduce far worse errors than not using the "best possible" method at some stage ??
For example, if you were analyzing the content of the images, you might find that (say) 1/3 of a midsection slice volume is occupied by fat due to a "beer belly". Fat is heavy, yes, but only roughly half as dense as bone, so failing to make a "fat vs bone" mass correction could be equivalent of getting the volume wrong by 15% or more.
There is a saying: "Measure with a micrometer. Mark with chalk. Cut with a chain saw." Your insistence on the "best possible" methods is like measuring with a micrometer, but your treatment of all volumes as being of equal mass significance is like cutting with a chain saw.
