Assignee:
You seem to be using assignedTo. I replaced this by assignee and it worked as expected: workItem.assignee = nodeHelper.getUserByEmail(replica.assignee?.email, workItem.projectKey)
Epic-Child:
Rather than use the Epic Link (that would use the issue key), use the parentId attribute (that will have the issueId). So, Jira Cloud outgoing replica.parentId = issue.parentId (usually part of default outgoing script)