Having some issues with motion capture data c3d files and mapping to actor

Hi all!
I found a bunch of c3d-files, captured 20 years ago at school and thought I’d actually make use of them.
So it’s my first time ever opening MotionBuilder. Been watching tutorials, but there are still things I’m confused about.

Uploading 2 videos to explain my questions.

video 1:

I’m not sure how to fix this mocap data. as you see, the points are named incorrectly and it doesn’t seem to work to rename it and save the c3d file.

video 2:

c3d file imported on actor file. this is my base t-pose file I managed to set up.
But when importing this data, as you can see - the left lower arm drifts off in space.
What can I do about this? is there a fix or is it an issue with the data I just have to live with and just fix with manual animation?

And here’s a zip with the source files hoping I’m lucky and anyone wants to help out =)

Thanks!