Existence is not the property that all things that are not made up have. There are an uncountably infinite amount of conceivable universes which have not been conceived and also don't exist. You're confusing "not X" and "the (phenomenological) opposite of X".
Well, when we say that anything which can take the value x is a something, we also imply "there is x".
So you would say numbers exist? "Five exists" sounds like a type error to me - it's a mathematical concept, not an object.
X having the value of a something is sufficient for the truth of "There is X."
I don't get what you're trying to say here.
So the reason that it can't be that there is nothing, is because "nothing is" is English's short hand for negated existential quantification, so of course it is never positively quantified.
You've befuddled the question, not dissolved or answered it. While I don't follow your reasoning exactly, it sounds like an argument against the validity of the null set.
I hope you're somewhat mathematically inclined, because what follows below is an attempt to express what's wrong with your reasoning:
Consider the program
if(N=0){ return 1};
where N is the sum over the array n_a containing the amounts in existence n_i of all objects a_i which belong to the set A of objects relevant to the question, and the returned value is the truth value of the statement "nothing is A".
For example: Let A = the set of all circular squares. Then A = {null}, n_a = {null} N = sum n_i over all i = 0. Therefore "nothing is a circular square" is true.
"Why is there something rather than nothing?" then becomes "Why is n_i not equal to 0 for all objects a_i?". (note: Tegmark multiverses can also be considered objects).
But that is all arguing from definition, which I know is silly when you are dealing with finding a truth, but what about when dissolving a question?
When dissolving a question you are trying to find the truth. Specifically, you're trying to find the true state of your mind which caused the question to arise. When you start using definitions, you don't look at your mind anymore.
As for the original question, I obviously don't have the answer, but a path that sounds plausible to me is that, information-theoretically, "everything existing" and "nothing existing" are identical: a fully connected graph is the same as a fully unconnected one. Humans don't think this way naturally because there's a physical difference between connected and unconnected neurons, and because we're working solidly in the low-level-of-connections part of the spectrum (only 10^16 out of (10^12)! possible connections are present in the brain).
So if it turns out that Tegmark's Mathematical Universe Hypothesis is correct (that the existence of all mathematical objects is consistent with our observations), then the question would be dissolved; there would be no mathematical rules which work upon mathematics itself to determine which parts of it "exist". The only question would then be "How is the MUH consistent with our observations?" - which is a mathematical and physical problem for which a clear answer exists. (If you prefer "existence" to have a use in language still, because it's a word, and words are used and defined by human consensus, and because by intuitive usage the Invisible Flying Teapot does not exist even though it's clearly a mathematical object, you can use "existence" to refer to things which can affect our universe, or something like that).
Where the mind cuts against reality's grain, it generates wrong questions—questions that cannot possibly be answered on their own terms, but only dissolved by understanding the cognitive algorithm that generates the perception of a question.
One good cue that you're dealing with a "wrong question" is when you cannot even imagine any concrete, specific state of how-the-world-is that would answer the question. When it doesn't even seem possible to answer the question.
Take the Standard Definitional Dispute, for example, about the tree falling in a deserted forest. Is there any way-the-world-could-be—any state of affairs—that corresponds to the word "sound" really meaning only acoustic vibrations, or really meaning only auditory experiences?
("Why, yes," says the one, "it is the state of affairs where 'sound' means acoustic vibrations." So Taboo the word 'means', and 'represents', and all similar synonyms, and describe again: How can the world be, what state of affairs, would make one side right, and the other side wrong?)
Or if that seems too easy, take free will: What concrete state of affairs, whether in deterministic physics, or in physics with a dice-rolling random component, could ever correspond to having free will?
And if that seems too easy, then ask "Why does anything exist at all?", and then tell me what a satisfactory answer to that question would even look like.
And no, I don't know the answer to that last one. But I can guess one thing, based on my previous experience with unanswerable questions. The answer will not consist of some grand triumphant First Cause. The question will go away as a result of some insight into how my mental algorithms run skew to reality, after which I will understand how the question itself was wrong from the beginning—how the question itself assumed the fallacy, contained the skew.
Mystery exists in the mind, not in reality. If I am ignorant about a phenomenon, that is a fact about my state of mind, not a fact about the phenomenon itself. All the more so, if it seems like no possible answer can exist: Confusion exists in the map, not in the territory. Unanswerable questions do not mark places where magic enters the universe. They mark places where your mind runs skew to reality.
Such questions must be dissolved. Bad things happen when you try to answer them. It inevitably generates the worst sort of Mysterious Answer to a Mysterious Question: The one where you come up with seemingly strong arguments for your Mysterious Answer, but the "answer" doesn't let you make any new predictions even in retrospect, and the phenomenon still possesses the same sacred inexplicability that it had at the start.
I could guess, for example, that the answer to the puzzle of the First Cause is that nothing does exist—that the whole concept of "existence" is bogus. But if you sincerely believed that, would you be any less confused? Me neither.
But the wonderful thing about unanswerable questions is that they are always solvable, at least in my experience. What went through Queen Elizabeth I's mind, first thing in the morning, as she woke up on her fortieth birthday? As I can easily imagine answers to this question, I can readily see that I may never be able to actually answer it, the true information having been lost in time.
On the other hand, "Why does anything exist at all?" seems so absolutely impossible that I can infer that I am just confused, one way or another, and the truth probably isn't all that complicated in an absolute sense, and once the confusion goes away I'll be able to see it.
This may seem counterintuitive if you've never solved an unanswerable question, but I assure you that it is how these things work.
Coming tomorrow: A simple trick for handling "wrong questions".