Absolutely. I worked a codebase that was littered with ticket numbers for a few years; many members of the team religiously added them but never read them. How do we know? Self hosted issue system and access logs. New starts would read a couple and that was it.

A ticket is a really opaque way of showing something. If you click the issue you're now going through several tangentially related comments and a few MR back and forths just to work out if its even useful to you. People will stop bothering to even open the link unless they are really stuck.

Whack a summary comment and then by all means include the link, but the comment is what most people will use.

Yes. Unless your explanation is thousands of words (which it probably shouldn't be), just inline it. I only speak for me, but if I had to look up (potentially multiple) Jira tickets just to understand what was happening in a source file, I wouldn't be at that job very long.

Absolutely. All that information dies when your organization migrates off of Jira years down the line and the issue ids no longer exist.

That depends on whether or not you think the Jira ticket will outlast your code or not. If the company will definitely never replace Jira then no. If there is a chance that they will, and for your sake I really hope there is, then you'll need a better comment.