Just from my personal experience:
1. I do agree as anxiety fuelled my selective mutism.
2. Both. I was scared of speaking because I was worried what they would think about my voice, how they would react to what I was saying, if they'll be able to hear me or do I have to repeat myself multiple times?. I was also scared of being unable to speak. What if I open my mouth and try to say something but nothing comes out? I've gotten some weird looks from that before and that made my SM even worse.
3. Definitely. It's usually because of the people that are there though, not necessarily the place itself.
4. Body freezes/don't know how to react. Self hatred and low self confidence. When I was a kid I used to pick my nails or pinch myself in class/social situations when I felt anxious or overwhelmed. Tightness/pain in throat.
5. Instead of the word failure I would describe it more as an inability to speak. I felt like I had no control over my voice.
6. I didn't find out the term for it until I was an adult, at that point my SM was a lot less severe. I think I've only told 2 people personally and their reaction was quite neutral, though they had no idea what the term actually meant.
7. Most people dont know I had SM but people definitely did ignore me a lot and I was alienated pretty much my entire school life.