Neptune, Poseidon, Danu, etc.

Sorry for rehashing an old thread, but I wanted to see if I could get some
clarification on the "original compound word".

Please let me know if you think any of what I write here is way off base.
I would greatly appreciate it. Also, I'm not up on my endings in IE or
OGr, so any corrections there would also be wonderful.

TiA.

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Breaking it down into what would seem obvious to me:

IE *nepot = grandson (later "female's nephew", if I remember correctly)
IE *poti = lord/master/powerful (with the horse association in there
somewhere?)
IE dhen/danu/? = waters (the "heavenly" dew/wet/water mother goddess)
IE *dheu = to flow
IE *dhen = same thing, in that it specifically designates the goddess,
or does this exist at all?

I see "Danu" (or, at least, Dan*) in the Celtic languages, in Slavic,
Sanskrit, etc. to designate, loosely, the 'goddess of heavenly water'. Did
all of these have the same change(s) from IE's *dhen (assuming that was
actually the right word)? It seems odd to me that would be the case, so
therefore I would assume there was some type of *dhen thing going on before
the respective linguistic splits occurred - perhapsn *dhen, or even a
better *dan-?

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G - Poseidon L - Neptunus

Now, someone mentioned the possibility of *Potis-Da:nuom. Is -om on
Da:nuom supposed to be the genetive ending in IE? And, if so, what is the
-s ending for on Potis? (And why wouldn't it be -ei or -e or something of
the sort?) Would it be reasonable to think that the full version of that
would have been Nepotisda:nuom?

Nepotisda:nuom > Neptunus
In Latin, why did the "usda:n" part change to "u:n" and not "a:n", such as
Nepta:nus instead of Neptu:nus? Is that a regualar sound change from IE>L?

Nepotisda:nuom > Poseidon
In Greek, is there any reason in particular why the "ne" would have been
dropped?
I can see how t>s is entirely ordinary, so I don't have a question on that
part. It is assumed, though, that the "potis" changed to a "pos*" and lost
the "is" (in whatever order), right? Or is it assumed that the "t" just
dropped out to form "po*s"?

I think something along the lines of Poseidahon was also suggested. If
someone could remind me again, why the "dahon", "daon", "danuom", or
whatever it was? Is that what was derived from the IE *dan*/*dheu/*dhen word?

I hope at least part of that made sense, because I'd love to figure this
"lord/powerful grandson of Danu/the waters" thing out. Unfortunately,
though, I'm still very new to this writing-out-linguistics-on-the-'net thing.

Lucida caela, quamquam vacillo,
Eris