The whole attitude of the Onyx Press seems to cater to uber fans...
At least as far as the 20th anniversary editions are concerned, this is pretty much exactly what is going on. They're compilations/love-letters-to-the-fans that try to bring together the previous three editions under one roof. As you've seen, this is somewhat unwieldy. The writing style in M20 is quite florid too, so it's not always a book that's easy to leaf through in order to find what you're after.
As you've seen, there are some good NPCs in Gods & Monsters. If you want to look at older editions, there are a few sample mages in
Guide to the Traditions, assorted mortals and critters in
Destiny's Price (stat blocks for these were repeated in
Tales of Magick: Dark Adventure). There are sample characters in all of the Tradition and Convention books as well.
As for adventures, there were about a dozen in total, but only three were stand-alone products - Loom of Fate, Chaos Factor, and
Angel of Mercy. Loom of Fate is quite interesting with lots of detail and dilemmas to work with. Chaos Factor is a batshit insane crossover with Vampire and Werewolf - good fun but completely over the top. Angel of Mercy was the first adventure for Mage and is a simple affair about stopping a demon summoning - I quite like it.
When it comes to building a chronicle on a general level, you might want to check out
Book of Mirrors and the
Storytellers Handbook. These are, respectively, the GM guides for Mage 2e and Mage 3e. The former is very 90s in feel but has some solid thematic advice on building a Mage game, running a chronicle. The latter is more rules-oriented and gives lots of interesting ways to hack and mod your game. Both are good.
Yes, people do play and run Mage

. It is easily the most challenging game I've ever run but when it works it's very rewarding. I feel your pain, though. I bought the game, tried to run a session and it was a disaster. It took me eight more months of reading and prep to feel ready to start again. Not the most accessible way to get into a game at all. Worth it, though.