My concept of these definitions is - professional means someone who makes a living off music, and it can be great music like Bernard Herrmann (in film music) or wretched music that somehow fills the commercial purpose (like a hack in the 1940s who plagiarized Rachmaninoff phrase by phrase because the producer wanted something "Romantic"). Amateur is anyone from a rank newbie to a great artist like Charles Ives who HAD to be an amateur because his music was too advanced for people during his lifetime and he couldn't even submit it for approval to the establishment, let alone make any money off it. Hobbyist is a person who simply enjoys music and may create something very good or bad, but not necessarily either; however, other musicians would be incensed to be called "hobbyist" as it implies they are not truly dedicated to their work as a drive for meaning in their lives, which comes from elsewhere, i.e. : selling insurance, working at a pharmacy, making contract killings, etc . A dilettante is a complex term, as it has referred in various eras to exceptionally talented persons who don't need to do something but do so for the mere fun of it - perhaps creating something wonderful - or to shallow buffoons who are merely screwing around.
Concerning Varese - he is beyond any professional, because he is a genius who created a new musical language. That cannot be classified in any group as each person who has done this is in his own classification.
John Williams I admire greatly because even though I loathe some of the films he scored, he was always trying to create interesting musical ideas for whatever he was offered, and he achieved a style of his own despite the snarky critics he has amassed who claim he plagiarizes - he does not truly plagiarize (and don't post examples of what sounds similar - thanks but I already know all of those) but rather, he has his own beautiful and powerful style influenced by the composers he knows in detail and loves, and it is a style which other people have now plagiarized.
I mostly agree with William and Errikos and have one more aspect of this discussion to add. In many professional fields there are some kind of licensing and objective educational qualifications required. A doctor can't be an "amateur"; a doctor has to go to an accredited medical school, pass certain state tests and qualifications and even do post-medical school work as a residence in a hospital setting before being able to hang up a sign and say "I'm a doctor". Similar with lawyers, stock-brokers, airline pilots and financial advisors. NASA isn't going to hire someone to write software for controlling satellites that fly out to distant planets or manage the assembly of spacecraft without that person having a PhD in physics or engineering, etc. Not that there won't be quacks or unethical people who fall through the cracks, but in general there is some organized, official method to determine if one has the ability, education and qualifications to practice their profession.
Obviously not so in the arts. There are no qualifications to be a film composer or TV composer, other than having someone believe that you are capable of doing the job. You might have a degree or three in music, but then again you might not have any degree at all. You might be a virtuoso player but then again you may just be adequate on your instrument. There is no licensing, nobody sits on state boards to check periodically if a composer is keeping up with the latest developments in composition. These facts contribute, I think, to the inability to draw a clear line between the amateur and the professional.
In earlier times a professional was recognized as someone with superior knowledge or skill, someone who could profess to know something about a subject. Before that the term even had religious implications in the church. But in today's ultra-capitalist world, the term simply means one makes money doing it. So we have professional sex workers, professional drug dealers, professional salesmen, and I do not mean this disrespectfully, only that we've limited the term to involve earning money at something. Whether one is really good at it, or has mastered their skill set and has a broad and deep knowledge of their subject seems mostly irrelevant in the arts.
I am not sure what Macker's motivations are for taking the positions that he takes, I also don't know if he might entertain some illusions about what professional scoring is about or whether he's ever made a living composing. Perhaps necessity is the mother of invention, but personal commitment is the father. Sometimes an artist is driven by the need to make money and survive and sometimes that artist is already surviving and prospering and is seeking a quality of expression that doesn't quite fit into the what the marketplace is looking for.
It's easy to get addicted to success. We always want more of it. Lily Tomlin once said if a person wins the rat-race they are still just a rat. A mature, wise and discerning individual learns both the meaning of success and the human values that underlie real success. If fame, money, status and power are the sole criteria for judging success we will find our world increasingly selfish, ignorant, divided, violent and dumbed-down by propaganda and lies. And that makes it harder for each one of us to be happy.