After a deep dive into Jinja2 and Pipelines, I finally figured it out. phew.
Hopefully this helps someone down the road.
Please see attached:
For the pipeline, I just had a trigger if a child record is added, updated, or deleted. Then search for the related parent record. Loop on step B search for, for each parent found update the parent record.
update [CList Master (Pipeline generated)] field using this code:
{{ b.clist_reformatted.split('.')|unique|join('.') }}
Just a note:
b.clist_reformatted can be seen on my screenshot and is a reformatted formula text that splits up the multi-select text.
This setup now allows the user to see an embedded report where only field values that aren't null on any child record will appear.
------------------------------
Shane Miller
------------------------------