I think the T pose is wrong because when I have make the rig with mixamo the original mesh don't have the normal T pose.
The original model is like /\ pose without bones.
The default pose with bones for this model is saved like this /\ not T
In the zip you can find the original mesh in obj format without nodes and bones.
I have try to fit the mesh and bones this way but it is surely not the best fbx model.
In my importer I have seen too, When I scale all matrix very low the mesh seen to fit better with the bones.
I'm not sure why some nodes is saved with the fbx Rotation and PreRotation name tag, Maybe this name only come from a bad export save i'm not sure.
I need to test more model types, Currently I can animate this model and simple md2 models.
I have try more models but all use similar bones setup, I need to find a model create from a other tool and not from mixamo about the bones.
I don't have test model from exemple: makehuman app but I think similar model use different rootnode parent, child method at the creation.
Currently I can load and animate model when I use assimp tools, but when I save the data in a personal format I get a lot more problems in the animations parts.
I don't not have get time to test more, It is very snowing here this week I have need to remove all this snow plus the other normal life things.
I try to work more on this tonight and this week-end, It's long to debug and many things can become wrong or cause problems
It's why here I try to have a little bones editor and use always the most possible the same bones setup with all models.
I can see and test more the newton sdk demo tonight.
Edited: I'm not sure if it is the same with your importer code, With assimp the matrix is transposed.