To get to the compare view, append /compare to your repository's path. Every repository's Compare view contains two drop down menus: base and compare
To compare the commits in timeframe you can do type the branch name in the compare dropdown, followed by a @, and then the date wrapped between a { } notation.
Here are two examples:
Date Compairson
Time entered in Weeks
Source: https://help.github.com/articles/comparing-commits-across-time/
Commits Done Using GitHub Web Interface:
Commits are commits whether you do it from the git interface or web interface. I guess there is no way to identify it.
Commits done by each developer:
It is clearly mentioned in the link you provided:
Contributors