Unable to flip a clip horizontally in iMovie 10.1.6
If I understand your question correctly, you can do what you want as follows:


Select the clip
Click on the icon you called the 3 overlapping circles
Click on the Clip Filter option
In the Choose Clip Filter window that appears, select the Flipped filter
This will now film your clip horizontally


I've created a visual diagram of steps 2 - 4  below: