How a post gets approved
Write the post and send it for approval instead of scheduling it straight away. The reviewer opens a private link, sees the post the way it will appear, and either approves it or asks for changes with a note.
Change requests stay attached to the post, so whoever edits it sees exactly what was asked. Once it's approved, it publishes at its scheduled time. Until then, it doesn't go out.
A portal your clients will actually use
Clients don't need an account, a password or an app. The review link opens a simple portal with your workspace's brand colour, listing everything waiting for them.
If a reviewer is slow, you can set an automatic approval window. Posts still pending after that many hours are approved on their own, so the schedule keeps moving.