Is there any best practice to add a button on the form to Force completion (reject) to cancel all the tasks and notify the requestor about it?
Please see below steps that need to be performed in order to achieve the same.
1) Create a field called "IsFormCancelled" or similar
2) Build a "Terminate Process" button or something similar.
3) Put a rule on it; to set the IsFormCancelled field to a specific value then submit the form
4) After that user can handle the logic of the workflow with the "IsFormCancelled" field and end the process.