A few questions about glass
Q1: A volcano consists of many lava flows that occur over a long period of time.
In general, only very few of these lava flows have the right composition and  the right cooling history that are required to form obsidian.

This is why only parts of a volcano consist of obsidian, and by volume it's a generally a very small amount.

Q2: And yes there is amorphous ice. It is known since the 1980s. These days it is being used in laboratory environments, e.g. for electron cryomicroscopy. This is a high-tech-method for observing biochemical reactions and cellular processes in situ. The microorganisms need to be alive and in the middle of a reaction when shock-freezed. Researchers want to image atomic structures of the biomolecules, not from ubiquitous crystalline water so it better be amourphous.

Q3: Glass from windows and bottles contains more Na and K than most Granitic rocks. Na- and K-rich ingredients are being added as fluxing agent (limestone, soda), in the glass factory, decreasing the melting point of the silicates and therefore lowering the production costs.  

I think a melt of bottle glass, when crystallizing slowly would initially form the minerals Nepheline and Leucite, but late in the solidifcation process also Alkali Feldspar, and some Quarz, among other minerals. 

If you cooled it so slowly that no glass phase forms, you still might get zoned crystals, having with a different composition from core to rim. That depends on the absence/presence of impurities and local chemical (dis-) equilibria.