Yes. POT file is provided as a base for translation in the root folder for each plugin. Translation files should go into languages directory, and WordPress must be set to your language to load the file. As an alternative, translation file can go into wp-content/languages/plugins directory.

The file name for translated and compiled MO file must be the same as POT file with the addition of language code: {plugin-name}-{language-code}.mo.

You can check out this for translating plugins using POEdit program:
wp.dev4press.com/tutorials/translating-plugins-or-themes-using-poedit

Yes. Plugin must be activated as Network Wide.