Usually the versioning for marketplace add-ons are independent. Use what ever versioning that makes sense to you. They are really only used in the “change log” section of the post.
That’s handed via the title of the topic. For example, here’s the title for one of my rule templates that only works for OH 3.2 through OH 3.4. " Time Based State Machine [3.2.0;3.4.9]. Here is the same rule template rewritten for OH 4: Time Based State Machine [4.0.0.0;4.9.9.9].
The version ranges somewhat follow the Maven syntax where ] means something different from ') for example. If you include the version ranges in the title, the add-on will only appear in the add-on store for versions of OH that fit in that range.
Unfortunately, when a new major version of OH comes out (5.0 is being talked about as the next release after OH 4.3 is released in a few weeks) that usually means breaking changes which means you can’t have one jar file that will be compatable with both OH 4 and OH 5 so you’ll need to create a new post. That’s not guaranteed to be required but it is likely particularly considering that the move to a major version corresponds with moving to a new JRE version.
I can’t help with the testing on earlier versions question.