Not in the least does the viewer realize X's real motive.

Both these examples seem unidiomatic to me.

"not least" is typically used in this grammatical context.

Most often it's used in a sentence like this:

I want this house, not least because it has a red phone box outside
(http://alanbaxter.posterous.com/i-want-this-h... )

"It pays to know the enemy - not least because at sometime you may have the opportunity to turn him into a friend." -Margaret Thatcher (http://raachelann.blogspot.com/2011/11/it-pay... )

Froggels' interpretation is also possible, but of course it has a different meaning.

Thank you. How about "Last but not least" instead?

Yes, that's very common, although it's a bit cliched.

http://dictionary.reference.com/browse/last+b...

http://idioms.thefreedictionary.com/last+but+...

Personally, I would not use inversion after "last but not least."

I can't make heads or tails out of your sentence, Julie4. what are you trying to say?

The viewer realizes not least what X's real motive is.

I think the above is idiomatic.
IMO there were two mistakes in the original sentence, viz.
1. the placement of "not least" was wrong, and;
2. "realize" should not have had "motive" as direct object but should rather have introduced a dependent clause.

#8 is an improvement, but it still needs commas to set off 'not least' as a sentence adverb. And I don't really think putting it in the middle of the sentence helps.

>>here meant as nicht zuletzt - after an enumeration of things the viewer finds out

Like this?

We discover that X is the long-lost son of Q. It also becomes clear that X has loved Y all along. Not least, viewers realize what X's real motive is / understand X's real motive.

If that's what you mean, I might still prefer something else, like 'above all' or 'most importantly,' but 'not least' is also possible. 'Last but not least' is, as eric says, too clichéd.