I had more typed but apparently I'm limited as a newbie (even though I've been a  lurker for years) so here's the one line summary:

If an 80% lower is partially completed enough to  legally be considered a firearm, can a licensed gunsmith then modify it to 100% and legally not be considered the "manufacturer"?

Any tested cases similar to this that anyone knows of?

I know build parties and manufacturing an 80% lower into a firearm for another person are not legal, but that's not what I'm asking.   I don't think the "80%" standard is written in law or regulation and my guess is this is exactly why.  Either something is a firearm or it isn't, but my guess is the ATF wants to have its cake and eat it too by not being overly specific.

Nope, once it's 81% it is a firearm.

Doing it yourself is all part of the challenge, A Jedi isn't a Jedi until he builds his own Light Sabre

Build parties are fine. The key issues is "in the business". As long as nobody involved is in the business, you are GTG.

https://www.atf.gov/file/11711/download

IME, most gunsmiths will not touch it though if it is not in a working state.

I like that.

No ATF regulation mentions "81%". It's either a firearm or it isn't.
Read this ATF FAQ's on receiver blanks

Precisely.

And these fine folks have been selling this product for years: https://www.americanspiritarms.com/product-p/asa-15-lower-95a.htm