Quote:
... The second makes more grammatical sense as "ever" is being used to parenthetically refine "never" (and specifically isn't a second adverb for "assume", since "never" and "ever" are antonyms)...

Hmm...
How about 'Never, never ...' (no comma, repeated as with very/very)
As you said, never and ever (being opposites) require a different pattern of grammar than does a word repeated for emphasis.
I yield.