For the first part, I just wrote down the responsibilities. If it benefited/helped you then that's more for the personal statement and/or supplementals IMO.
For the second part, what I did was I counted to total hours and then divided it over the amount of weeks. For example, if I volunteered 20 hours and there were 10 weeks in that semester, I said 2 hours per week. Make sense?
2- It sorta depended on the situation. If it was like a 1-week thing and I worked 60 hours, I put 60 hours per week. But sometimes I did what DrLTD did. So do whatever's the most reasonable. Adcoms have seen this 10000000's of times so I'm sure they won't get confused