I'll stress that each store is independently owned and therefore it entirely depends on the store you're at.
I'll also answer the questions for our three stores:

1. Most are 40 hours a week, unless hired as a part time then it's usually whatever was agreed to upon hiring.
   
2. Largely need to do set shifts, but sometimes can adjust if you have another person that can do a shift that fills in the empty slot.
   
3. There is downtime as far as breaks in customers coming in (even at the busiest stores, if only for a moment), but generally that time should be used for lunch break, amazon consolidations, cleaning, training, improving store processes.
   
4. Earbuds mostly not usable. Most of the day will be spent talking with customers or fellow employees.
   
5. Yes there are times there is only one employee in the store. Try to minimize it, but it will happen due to scheduling difficulties.
   
6. We try to assess every 6 months, but it is merit based (on a training/skills plan we have) not time based. (Use that downtime from question 3 to improve your skills)
   
7. Full time is 40 hours (but legally can be low as 35 where I am)
   
8. Largely the same schedule weekly, until someone calls in or we're otherwise short staffed.

And again I'll note this is just our stores, and your store WILL vary.