Is it still possible for an amateur, a hobbyist, or a science buff to make relevant discoveries in chemistry today?
It's conceivable that somewhere out there, someone with a completely fresh point of view could have an annus mirabilis in an area like theoretical chemistry, working with just a paper and pencil (or a laptop) and home-brewed equipment and resources.  Yes, for most of us, all of the low-hanging fruit is gone---but for a real giant, the whole tree might be full of low-hanging fruit. 

Most of us aren't giants, but as Newton said, we can stand on the shoulders those who are. Doing that requires some ability to search the extant literature, even for "small to moderate contributions". 

In addition to the obstacles you've listed above, there is a third impediment: paywalls. 

There was a time when you could walk into a university library and sit down with the Science Citation Index and do a good job of finding research related to yours. Nowadays, you need a faculty or student university ID to do such a search and retrieve the papers you need. Without an ID at a good university, our amateur, hobbyist, or science buff will  hit paywall after paywall. Amateurs might have to pay half a day's wages or more for the pdf of a single paper! 

Paywalls are not only a major obstacle for amateurs---they discourage professionals in the third world, and they shut out young people who are beginning their careers outside of university.

Consider Jack Andraka's story, here: http://blogs.plos.org/thestudentblog/2013/02/18/why-science-journal-paywalls-have-to-go/