If a user has done long term delegations for his/ her task, is it still possible to respond by himself/ herself when required?
Or it can't be done forever?
I do not think that the original person can respond as the task is now secured using the new person user name.
The way SharePoint works (and thus Nintex tasks) that once a task is assigned to somebody else they are the owner. And unless the person is an Admin they would not be able to respond to the task as Rob Pratt was saying.
In the past I have built a Site workflow that can handle the re-assignment back to the original owners. This could be carried out by a manager the task owner.. Anyone really.. Here is a blog post that sort of goes in that direction, but exactly what I was just talking about.. ! Confused The blog post just highlights how you can utilise workflow to re-allocate tasks even if you are not the owner.. http://www.sharepointpub.com/nintex-workflow-different-ways-to-delegate/