Understandable. I absolutely would still recommend preparing for an upgrade in the future. When applications go End of Life, it can cause many serious performance and security issues if you remain on that version. This can also cause compatibility issues with current technology, such as TLS version, your web browser, and more. At this time, you have time to consider which add-ons will stop working, and their replacements.
As for your second question, Confluence spaces aren't hierarchical. They're all on the same level. This question is also unrelated to your original question, so I would recommend raising a new question and describe exactly what you need and perhaps one of us will be able to recommend an add-on!
It's certainly worth trying, but Jira 6.0 has been End of Life for some time now, so I would foresee issues when linking with the recent version of Cloud.
Have a look here for how the link would need to be set up:
I would advise you to upgrade Jira to a supported version, which you can find from our Atlassian Support End of Life Policy. That way you can ensure there will be no issues.