...Why is there something instead of nothing?...
This sounds like the begining of an apologetic for the existence of God to me...
The interesting conclusion of this ultimate puzzle is that, we can be sure of, it that at least something exists.
Norman Geisler used this as his starting premise for his version of the cosmological argument. More specifically, he used the undeniability of his own existence as his starting point.
...Many physicists search for the most elementary laws of physics, and believe that a law is more likely to be true, when it is simpler, more elementary.
Occam's Razor?
Some think that at some moment, humans will understand how the Universe and everything works, and, even more, that we find out why the Universe is necessarily as it is.
Ah... Theory of Everything!

...I believe humans cannot ever give a satisfactory or final answer to this ultimate of all questions. Why is there something instead of nothing?
I think I have a good answer to this question, and I personally find it satisfying. I'll get back to this in a moment...
...Nevertheless, one thing we can be sure of: this nothing is not correct: we do not have “nothing”, but definite and absolutely do indeed have ‘SOMETHING’. This shows that the simplest model is not always the correct one.
I'm not sure I'd agree with you on this point. I certainly agree that something definitely, absolutely, undeniably exists, but the fact that one can conceive of a simpler model here (non-existence) doesn't prove the principle wrong, IMO. In fact, haven't you already established that we can't truly conceive of nothing anyways? How could we say that non-existence or nothingness is a simpler model if we can't even truly conceive of the model in the first place?
The universe is almost infinitely complex and to me this points to the simple logic that it is the creation by an infinite, intelligent power. Nothing is the very most basic of all concepts and if there were nothing, there would be no creator, of course.
Now we're getting somewhere...
Some people may argue that the universe was created in the Big Bang ( but whom and what pressed the button of the big bang in the first place, so to speak?) , and that positive matter and positive energy are actually negated by the simultaneous creation of negative matter and negative energy. However, this doesn't answer the other question, where do matter, energy and laws of physics then come from in the first place?
As far as I understand it, these things break down at the singularity anyways, don't they?
...If something exists because it either was a modification of something or else, Something or Somebody else created it, then what caused that to exist?
Ah... now you're alluding to an infinite regression of finite causes. There's a subject that'll make your head spin if you think about it too long! An infinite chain of finite causes can be likened to a chain or series of dependencies, each dependent upon something else for its being or existence. But even an infinitely long string of dependencies can't exist independently; it would have to have been derived from something that isn't part of the chain. That something would have to be independent and underived; that is, it would have to be self-existent.
It seems that our logic is unable to deal with the question
I think the question is simply misguided. The question
falsely presumes that nothing could have potentially existed instead of something. The underlying premise is therefore faulty.
That's my take on it anyways

-Michael