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b) Is there a natlang analogous to my system: any of /n S l/ in all  
and any of /n S l x/ in word-final codas?

Finding a natlang that has that inventory won't validate this
system, just as not finding one won't invalidate it.  It all depends
on how it came to be in your language.

In one of my languages, there's only one internal coda, a nasal
homorganic to the following consonant.  For final codas, though,
all coronals are allowed.  This results in /n/, /t/, /l/ and /s/.
However, there was a sound change that affected all non-nasal,
non-continuant coronals (here, I'm taking advantage of the fact
that /l/ is sometimes +continuant, and sometimes -continuant,
depending on who you ask).  It went something like this:

*t > T / _#
*l > K / _#

I think [K] is a voiceless lateral fricative, right?  If not, that's  
it went to.  Then a separate sound change deleted [K].  So, in effect,
you have codas of /n/, /T/, and /s/, and an internal coda of N
(nasal), even though you have a coronal inventory of /n/, /t/,
/T/, /s/, and /l/ (and, I guess, /tS/ and /S/, but those are the
result of /t/ and /s/ coming before /i/ and /j/).

"A male love inevivi i'ala'i oku i ue pokulu'ume o heki a."
"No eternal reward will forgive us now for wasting the dawn."

-Jim Morrison