Answering all your questions one by one:
1. There is no significant difference between Delete and Terminate. Terminating your instance essentially means deleting your instance. The instance doesn't immediately disappear from the list of instances in your EC2 dashboard but if the status shows terminated, then that instance has been deleted.
2. Since terminated instances are deleted instances, there is no use of them.
3. You can terminate the instance yourself, once terminated you will no longer be able to use that instance.