How can a previous stay in the U.S. help me get a waiver for the travel ban?

It would be one helpful factor. The other good news is that the Nigerian Foreign Minster who met with the U.S. Secretary of State indicates that they should be able to comply. So I am hopeful they will lift the ban, or grant a waiver.

The order takes effect on Feb. 21, 2020. It is not being retroactively applied and since you have obtained your EB-1 visa, this new order should not apply to you.

No single factor is determinative. In theory, the travel ban is in response to current circumstances and past vetting for a visa is thus somewhat irrelevant. However, your attorney would raise any and all available grounds under the waiver criteria.

It depends on the nature of the prior J-1. If, for example, you were serving as a physician in an underserved area on the J-1, you could use it to argue approval of the waiver national interest. There are several prongs to the test, for instance, undue hardship; the entry would not post a threat to national security; the entry is in the national interest.

No, that is not one of the factors that would qualify a person for a waiver.