The new JIRA integration is very helpful. It would be great that while in JIRA I have the option to add an existing test case. The add test case feature only lets you create new items.
Thanks for your feedback, added to the list of feature requests! You can also do this directly in TestRail and you would only need to add the JIRA issue ID to the References field. The JIRA add-on would then automatically show the test case(s) on the JIRA issue page.
This is also something which i was expecting from the Add Case button. But looks like it can be used now only to create a new test case.
Yes, the Add Case button opens the new-case form in TestRail. Linking test cases would be a bit more complicated since you would also need a way to search for TestRail cases in JIRA. We are happy to look into this for a future version of the add-ons. You can currently simply map the JIRA issues directly in TestRail by adding them to the References field of the relevant test cases.
I hope this helps!