I'm always intrigued by the use of music as a stimulator of the brain. I have personal experiences with music that address many of your points, 1-8, though I may have a question or two about a couple of your points.
As background for this experiment, I have played flute since I was 7 years old. And I participated in more than one singing group in the past. Just for discussion, I'd like to respond. Keep in mind that I'm just one data point!
1- Yes, I have found that music does relieve my own stress by redirecting my mood and thoughts. No argument there!
2- I'm not sure what type of "endurance" you mean, i.e., physical? mental? both? Can you define this term more specifically?
3- How does music make one healthier? Do you mean physically or mentally? I am physically healthy (at least!), but I'm not sure I'd attribute that to music exactly.
4- Singing with a group of friends (or just others who like to sing) has made me happy, really happy.
5- Question: How does playing a musical instrument make a person more successful? Do you mean in career? Or family? What kind of success do you refer to?
6- Please define what you mean by music making a person "smarter." In my case, I do find that playing an instrument has improved my ability to think "musically" over time. I am always running a tune in my head, almost on autopilot, sometimes analyzing it, sometimes adding variations.I understand tempo and can keep a steady rhythm. My sense of time passing is pretty accurate. But I'm only one person, so I would say this would need more "data points" to make this statement meaningful.
7- Over time, yes, I have found music to improve my memory. Consider, though, that I'm practicing memorizing small, simple tunes. So it may be that the practice of memorization helps improve memory. Would an actor (that is, someone who must memorize pages and pages of spoken lines in a script) claim that acting might also improve memory.
8- My mother has Alzheimer's. It's fairly advanced. Through the progress of this disease, I have noticed that she still resonates somewhat to songs. She can even recall a few words of lyrics from songs she'd known growing up. I know she recognizes some of the music they play at her facility. I can see it in her face as she taps her foot or moves a hand in time to the music. Other residents with dementia (there are many types of dementia) also resonnate to music.
Thank you for posting your hypotheses on music. They're intriguing. I hope others weigh in.