I cannot speak with any authority on Goth but as a long time member of another "subculture" that gets a lot of attention, I'd like to offer my insight.

Is it a Sub-Culture, a lifestyle, a label, or just an excuse to act different?
All of the above, really. There will be those who will only dabble in Goth. They will go out to a club for a "night on the wild side" and when it's over, they will go back to being whatever they are most of the time. Of those who "go Goth" simply to upset their elders, nothing need be said. These are the posers who are rebelling for the sake of rebelling. They haven't a clue about much of anything and aren't likely to until they stop playing dress up and get serious. "Lifestylers" have no real committment either. They may talk the talk and walk the walk very convincingly but are likely to discard Goth for a more conventional lifestyle once the Goth lifestyle begins to bore them.
Goth has been around long enough to qualify as a subculture and it is impossible, I think, to establish some sort of benchmark for what is Goth and what isn't. The first question that would have to be decided is: who decides what is Goth? Some self-appointed Guru of Goth? A magazine or website? An online organization of some sort? I'm thinking that it would be impossible to set a standard that would universally agreed to. Much like trying to set a standard for who is a "real witch." *sigh* Clothing does not necessarily make the Goth although wearing a lot of black can earn you the label "Goth" especially if you are in high school or college.
By preference, I wear black clothing from the skin out, black nail polish and silver jewelry. I have no fear of the dead or Death and the themes of death and transformation have been lifelong fascinations. Dead things do not send me running for the hills. [The result of a rural upbringing.] And yes, I DO love vampires and have since the 60s!  So, am I Goth?

Any time, DK.

It was to my understanding that most of the Gothic culture is mainly centered around the atmosphere of the music.
As a person who has delved deeply into the Goth scene let me add my insight.
There are blurred lines between what is truly goth, and what is minimally understood by the public mainstream.
First of all, I have not heard one Goth band on the radio, seen one goth kid on tv, or read one young adult book that has not in some way or another made a failed attempt at displaying or understanding the goth style.
Goth is first and foremost, a state of thinking, living, and being. It is a culture encompassing many different textures, atmospheres, understandings, ideals, and lifestyles.

Yet, many think that it is the style of clothing that defines the goth scene.
When truly, the music is responsible.
There are many genre's of music associated with the goth scene. just take a look at each of these genre's and you will understand the style and look of each one.

Gothic Music(Mors Syphilitica, Switchblade Symphony. Lycia, Rasputina) Unquiet Grave Compilations 1 and 2000
Industrial(Skinny Puppy, Wumpscut, Suicide Commando)
EBM/Synthpop/Futurepop(Apoptygma Berzerk, VNV Nation)
Ethereal(Cocteau Twins, This Ascension)
Shoegaze(Autumn's Grey Solace, My Bloody Valentine)
Deathrock(Christian Death, 45 Grave)
Gothic Rock(Inkubus Sukkubus, The Sisters of Mercy, The Dreamside))
Gothic Metal(Penumbra, Tristania)
Darkwave(Cruxshadows, Thoushaltnot)
Just about anything on Metropolis Record, Dancing Ferret, Cleopatra, Projekt, Cold Meat Industry, orSacrum Torch.

I disagree, not completely, but I do very much disagree.
I understand the old influences, they are very obvious (yet I disagree with you on the precise influences)
The movement was sparked by the music. Music such as Joy Division and blah blah blah, the first wave of gothic music. It DEVELOPED under the unfluences you mentioned, that imagery which has expanded much beyond simply gothic architecture. It now draws many influences from other writers such as Lovecraft, and artists such as Austin Spare. It has now become more otherworldly than it has ever been, and comparing it to very old and overstressed, undercriticized writers would be only beginning graze the surface of it's composition.
It is no longer about bringing only the ancient beauty to the modern world, it hybridizes that with modern and even post modern ideals. It takes the very attractive and much longed for older beauty and restores it, yet it blends with many new atmospheres. The word "Gothic" or "Goth" in itself is only a generalization. It is not about smoking ciggarettes and having a bad attitude, being rebellious, hating authority, and being different.
It is a place where almost another half of a world is created for the benefit of those who are attracted to it's ideals.
And I will always truly believe that it is the music that sparked it and keeps it alive. It is what holds the vast world of underculture together.
Would there be a Dark Cabaret style without Dark Cabaret music? Not an effectively inspired one.
You were right, the music is used to communicate. Yet instead, we used the music to create an atmosphere to build from. To create an understanding. The music was the basis and the inspiration, and will always be.

I disagree respectfully.
There can be many outlooks on how the Gothic Movement started, it's association with many other genre's of music, and it's roots.
I beleive the gothic movement is a vast world of textures, existing solely for those wishing to exist within a much ore romantic and characteristically dark atmosphere.

That can encompass many styles of music.
And if I remember correctly, I said that these genres were ASSOCIATED with the movement. Each genre creates it's own world.
That is how I view it, it and that is how I will always see it. I do not see the word "Gothic" as a technicality.
I will agree to disagree, but I would like to point out one thing.
Wether or not  the word "Gothic" was coined in this context by a critic at a Joy Division concert, the fact remains true that the movement didn't truly start until that time frame.