this is the area (415 issues) the IRS agents (at least at one ASPPA Conference) concluded made no sense.
supposedly you would count the 2017 contribution toward the 2018 415 limit because it was made after 10/15. But if you wait until 2019 to make the contribution (corrective contribution for 2017 under EPCRS) then it counts toward the 2017 415 limit. And of course, if someone quit in 2017, they are due the safe harbor, but would have no comp in 2018, so no 415 limit, which means no contribution!
so the 'rules' for 415 limit make no sense in regards to this area.