Jira importer - technical evaluation

With Jira Importer - Status quo discovery (#434808 - closed) we gained information on status of the currently available Jira importer.

That investigation confirmed that Jira importer is in an incomplete state as it doesn't import anything but Jira issues title, description, and labels only (see).

There's also a dependency on Jira issue integration that needs to be set up before it's possible to use the importer.

What this investigation issue should answer

1. Why current importer depends on integration? Is it possible not to have this dependency?
2. From technical POV, would it be better to implement an importer from scratch as oppose to continue developing the current code?
3. Which best practices (batched migrations, workers idempotency) used in other importers could be readily implemented in the Jira importer?
4. Roughly, what proportion of the total effort would be saved if we would base the Jira importer on importer framework, assuming the framework already existed?
5. There are Jira Server, Data Center and Cloud. Possibility to import out of Jira Server is the most urgent. Is it necessary to have 3 separate importers or is it possible to have only one? If one importer serving three sources is possible, would it be better to start from covering Jira Server only or would it be feasible to cover all three sources at once?