I decided to go in the direction that I did after considering...
Comments from two other language enthusiasts (who were at odds in their interpretation of double negatives).
My understanding of Mandos from what I've read in the Republic Commando books and support articles, particularly the general tendency toward pragmatism, straightforwardness, and uncomplicated thinking.
What I read in the Wikipedia article about double negatives, that more languages do not have negative concord than those that do. Article: https://en.wikipedia.org/wiki/Double_negative
The examples of multiple negatives in the Wikipedia article demonstrating that more negatives leads to less understanding and greater ambiguity.
Languages that have rules for negative concord as key component of the language (rather than using it for emphasis only in limited circumstances) have more complex grammars than Mando'a has.
The fact that we are not creating Vulcan or Klingon or even Taung. Mando'a users come from a variety of backgrounds, so the language should be easy to learn by the widest range of speakers.
What grammar rules from her English dialect that the author might have been trying to incorporate.
The mention in Star Wars Insider #86 that Mando'a is supposedly "easy to learn."
The mention in Star Wars Insider Online Supplement "Inside Mando'a" that the language is "a really simple, logical and colorful language that depended on word order rather than cases and other complex grammar" and has "No complex grammar."
How I could keep the rules as simple as possible while maintaining the spirit of the definition given for "nu draar."
I tried to image what the simplest way was that I could express the grammar rules for double negatives that reflect both the straightforward nature of the culture and the rule that the original author put into place (that a double negative can be used for emphasis), while keeping in mind that the original author was not known for thinking through consequences of grammar rules.
I attempted to create rules that required the least amount of explanation and the least amount of memorization of which words are negative and which aren't and of what different combinations of words mean. I wanted rules that produce meanings that are logically compatible with other Mando'a grammar rules. I'm sure I need to get even more specific than what I've done so far to make the rules even clearer.
While new words can be derived from non-English languages, grammar rules should not be, because individual rules in other languages are tied to other rules in those languages. This or that sentence structure can exist because these and those other rules or words or the like exist in that language.