2) It's optional to define parameter "EXCEL_TEMPLATES_PATH". If the parameter is not defined, you should put the templates in <PPM_HOME>/conf/custom_excel_templates.
The advantage of the parameter is that you can point it to a shared folder and maintain your excel templates in one single place rather than copy them to every node of your PPM cluster.
3) In next PPM minor version, it will be possible to use XLSM files (excel with macros) as Excel Report templates. Actually it's already working, but the files will be renamed to XLSX when the report is generated (and they must be manually renamed before the file can be opened). No such issue in the next version: If the template is an xlsm file, the report will be an xlsm file.