At first: I am not familiar with Chinese burial rites and their literature and in the context with Jade, but as anarchaeologist, I can sum up few examples of my own experience which belongs to excavation activity in central Germany.

I can tell you that the preservation of a buried body is influenced by several circumstances. Not all of them are recognizable through archaeological evidence. For example, it would have made a difference if a person died during summer or in autum, because the process of decomposition would have been much quicker in summer without artificial cooling. But it is rare that we can link a specific burial to a specific date. This might be the case for a king or queen, because it is possible there are surviving written sources which provide the exact day of death and the funeral, but this is rare.

As far as I am aware, if the body of the deceased should be preserved there is a complex process necessary to do so. The example which probably is best known would be mummification. Apart from the examples of human and animal mummification from Ancient Egypt, which was a complex procedure for which we do have written sources. This process can be achieved by accident due to special climate circumstances being present at death. An example for this would be the socalled "Ötzi" or "Iceman", a man from the Bronze Age who died whil crossing the Alps. He was found by wanderers around 30 years ago. (Museum website here: https://www.iceman.it/en/) But you asked specipically about intended processes, so I will focus on those.

So, when a body is buried, the wood used for a coffin will likely affect the decompositon. I talked to an anthropologist when researching an Early Medieval Burial Site, and she told me that acidic ingedients in some wood types can prolong the process a lot.

In addition, the depth of the burial and the soil itself do have a lot of influence. In Germany, in my region the main component of soil is silt which is mixed with clay. Geologically spoken, the silt slowly becomes the clay during erosion and accumulation processes. The deeper you dig, the more silty the soil becomes. The higher up, the more clay you will find. Apart from the substance of the soil, the silt has a high amount of chalk in it. The soil more on top will be affected through agriculture. The plants growing on top need chalk for their nourishment, slowly helping to turn silk into clay. As bone material has a lot of chalk, a skeleton is much more likely to persist deeper down. Of course bigger, stronger bones survive longer than smaller, more fragile ones. At my excavation site last year we had burials where it was hardly possible to determine that there had been a body next to a nearly perfectly preserved skeleton, which was just buried deeper. Same site - same priod of use.

The next good possibilyty to have bodies survive over a long time are bodies found in bogs, as there are special conditions cutting off oxygen which prevents the bacteria causing decomposition to work while additionally drying off the skin.

Now I am leaving my knowledge and turning to some speculation: As far as I am aware, Jade is highly sybolized. It stands for eternity and is - especially in prehistoric societies with a limited amount of tools - hard to work on. So Jade items are something special. So, what you desribe sounds to me like Jade was believed to help preserving the body - and maybe the soul? and because of this, it was used in some kind of burial ritual were - depending on the status of the deceased Jade items were put on the body. The differences you decribe in your books and finds then would come from very different environmental circumstances which would cause great differences in the conservation. So, bone crumbs with jade objects might be true, but conserved flesh might also be due to great conditions.