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Is it possible to let the commenters decide if they want the notification

In this version all commenters will receive the notification. In the next version it will be possible to check a checkbox to get the notification.

Why do I need a twitter account to use this plugin?

You need a twitter account to make this plugin work because the plugin needs a place to send the notifications from. If you don't want to clutter your twitteraccount with these replies, make a new twitter account specifically for this purpose.