I am using SP 2013 on premises. 15K Large list threshold.
The first solution returns a blank reverse lookup (no change from original problem).
The second solution returns “There are no related items” constantly. I assumed the second solution was to use the complete script that I had working for the other case (i.e., DFFS Cascading dropdowns with common text strings) and use the query above.
Note that I had two typos in my diagram. See attached Parent and Child corrections in BOLD text.