Reference:
https://help.salesforce.com/articleView?id=mc_jb_schedule_an_audience.htm&type=5

Explanation
To test different subject lines with a pilot group and then send the winning subject line out to the remaining customers, you should place a random split before path optimizer for the pilot group. This will allow you to divide your audience into two groups: one for testing and one for holding back. Then, you can use path optimizer to test up to 10 subject lines with the testing group and select the winner based on open rate or click rate. Finally, you can send the winning subject line to the holdback group.

The best text for a call-to-action button is clear, concise, and compelling. Option A provides the most information about what the user will get by clicking the button, while also creating a sense of urgency and value. Option B is too vague and does not specify what the user will download, and option C is not a call-to-action at all, but rather a statement.

To cancel the send after it was scheduled, you should navigate to the overview tab in email studio. There, you can see all your scheduled sends and cancel them if needed. You can also edit or reschedule your sends from this tab.