There is something better, it's called FileMaker Server, which gracefully handles filemaker backups without any manual process or cobbled together solutions. If your data is important, then it should be worth the investment to purchase server edition.
Question 1 - OnLastWindowClose is file specific, and takes place when the last open window of that specific file closes. So closing the interface file will not also close the data file.
script triggers process BEFORE the action that triggered them (in most cases). So if you use exit script result = false in OnLastWindowClose, it will block the window from closing. Otherwise the script evaluates BEFORE the last window closes.
Get(UserCount) is still a function, I am not sure who told you it was gone, but I just checked the FM13 documentation and it's there.