How do I bypass the need for the developer to edit a text file in Notepad++, fill in details, save the file and close Notepad++ before the diff is created?

I think the recommended way to resolve this is by using Projects. For my team of developers I have a project #team_r which are the group of reviewers and #team_s which is essentially everyone else on team who can subscribe.

When a developer fills out the text file (which is required) it looks like this:

Task ##### - Short title of changes

Summary:
 - Itemized list of changes
 - 

Test Plan:
Description of how the change will be tested

Reviewers:
#team_r

Subscribers:
#team_s

They can also add additional developers not on the team who would be familiar with the changes as well, giving input.

How do I attach the auto email events to a project / repository to send out emails when new diffs are created?

If you have the Mail application configured properly, the above process will email all the reviewers and subscribers. Most all the options are listed at /config/group/metamta/.

I still haven't figured out how to setup the actual mail SMTP settings for "PHPMailer - Configure PHPMailer" in the Phabricator homepage because it says that they are locked.

If you look at the config option, even though it says it's locked, it will tell you more information:

This configuration is locked and can not be edited from the web interface. Use ./bin/config in phabricator/ to edit it.

You can only modify the setting from the command line while ssh'd into the server.