Hi! Thanks for all the details.
The “halved sized” character is strange, while the not animating fingers could depends on the used animation: Mixamo allows you to rig characters with several fingers configurations, maybe the animation has been created with a different configuration. You should see it playing the original animation asset directly in the animation editor and checking if the finger bones are animated.
If this doesn’t help, PM-us the FBX files downloaded from Mixamo (character and animation) so that we can look directly at them.