I'm still having a hell of a time figuring this out. It's crazy, I think I've gone over every permutation I can.
I guess details may help further.
The Internal name comes from the URL right?
I know parts of it are working.
If I take out the %20 from "Customer Fabs" the error shows that it can't find the columns. That makes sense so I think %20 needs to be there.
If I take out the %5Fx0020%5F from "Customer Name" the nothing works, no error.
In the form I am creating when I have it as above it appears the customer list is fine and the child column is shortened to nothing. So I feel like it's working but isn't finding the relationship.
All of this is built with site columns and content types. Is that where my problem is?