Hi Markus,
At first, I'm not speak english, so sorry my mistakes in the writing :)
Next question is, how can I generate this email and send it to the user ? Do I have to write a task handler for sending this email and inserting the link and user's email address ?
In BPMN 2, to this type of task you must use a Service Task. Check Domain-Specific processes. jBPM provide a built-in task for this purpose.
How can I get back from the confirmation-endpoint (where the emailed link points to) back to the task to complete it ?
You must build a link inside the mail that have the taskId that you want to complete it. This link will forwarded to some service that pick the taskId and with Human Task Api you can complete it.
And finally - how can I include a ... whatever.... that automatically removes the pending confirmation tasks and assiciated data, if the user does not click the confirmation link within 3 days ?
In theory with Time Event Boundary event, but jBPM don't have a built-in persistent timer. You can vote JBPM-3170, however in the issue you can find a workaround.
Cheers,
Juan