[SOLVED by setting Minweight to 0.005f]
But why does it happen with the standard t pose too? And why isn’t blender showing this behavior as well?
The mesh is intended for use with the dual quaternion version, so this unoptimized mesh strangely looks and behaves much better in the DQ version than in the normal version.
But I will definitively look into it sometime in the near future and try out some optimizations.
Meshes, which have optimized topology for simple linear skinning tend to behave worse in the DQ version than “my” modified mesh. (have tried out every character creation tool out there and decided to go for MakeHuman+own weight paint mod+subdivision+morph target for armpits/upper arms.
I’ve seen meshes from Character Creator 3, MBLab or DAZ and they have a nice topology, but don’t behave as nicely as my modified mh mesh version+they have other issues.
CC3 is more optimized for linear skinning, while DAZ needs many morph targets to behave correctly and MBLab is still experimental and not optimized for gamedev as well.
The MH default rig has many bones, because it gives more control and it has been made for linear skinning. It’s always better to have more bones for better rig control. If you don’t touch them, they will not bother you.
As we have limited resources, not everything can be simulated perfectly and I’m still waiting to witness one perfect universal “out of the box” rig for many different randomly generated characters, which is suited well for use in a game engine.
As for reweighting: I know it’s fairly simple and blender has even the option to truncate values below a certain threshold, but that is still causing an uneven skin with different animations. Therefore the need for changing the minweight to a lower value.
I’ve set the value now to 0.005f at it seems to work at first glance 
And compiling time is only bad if you do it for the first time, but than it is just a matter of seconds. I guess it is a feature of the new VS Studio 2019.