I have been making my own scents for a while now. Thinking along the same lines you are I went through all my boxes and made a spreadsheet to identify the most commonly used ingredients. I found that Eugenol, Citronellal and geraniol among several others to be highly common. I purchased bottles of each...they were VERY INEXPENSIVE BTW. When I smelled them individually, they all are highly potent but far from unique. It would seem to me that many designers are making fragrances where they can base the major portions of the scent around these cheap aromachemicals and just season it with a few less common and more costly ones to make a new scent to bring to market.
For your second question, I think it may be posible to ID a scent on the timeline much the same way you can tell how old certain items are based on if they have a barcode or not, a patent number or not, etc.