To get around it I had about a dozen go’s and found a video where to get the A pose of the mannequin to the mixamo T pose it was basically raise the arms up 60 degrees, and then move the lower arms back 10 degrees.
Doing that the animations were close. Not perfect but close enough that in the animations themselves I just tweaked them a little more and rekeyed and got through it.
Retargeting is still quite painful in some cases. The fingers being all mangled still confuses me but I suppose its about persevering through the repose over and over until it works.
This poster also had a problem with this, and they posted a link to a video to fix the fingers in the top post! Take care, it’s the second video in the series and there is a link to that first vid in the description.
As another poster mentioned, you have to have your character pose exactly match your mocap pose. The offsets between source and target poses are really strong. If your mocap has a stick-straight neck and your target has a natural neck, the naturalistic mocap will be additive to the character, pushing their neck further forward than you want. Same thing with the fingers-- stick straight mocap needs stick straight character.
I had my finger chains defined as just the three finger joints and the metacarpals were left out. I needed to also go into the IK Rig, search for “meta” in the tree and right-click Remove. This stops the retargeter from putting wacky rots on the metas and leaves them in the offset aquired from the pose, as described above.