So glad you asked. Just happen to have some blog posts that may have all the information that you seek.
If not, please let me know what is missing and I will supplement what is already written.
Looking forward to learning of your progress.
I suppose you are looking for the buttons marked by a red square in the following image:
Move your CTI to the time where you want to put your points(keyframes), and then click on the center of these three buttons above to apply a point(keyframe).
Apply 2 or more points at different points and then move them up and/or down to do keyframing.
You can also hover over these points to see what each one of this does.
Although one can place keyframes along the video and audio track content rubberband as you describe with the CTI moves and Add/Remove button under the Video 1 name in your screenshot, I suspect that paulrudophmusic was thinking in terms
of Ctrl clicking point on rubber rather than moving the CTI and Add/Remove button.
Both approaches should work for placement of the Opacity keyframes that paulrudophmusic seeks.
How do I place points on this line and then adjust it up or down?
My habit is a little different and I use version 11.
Select a clip or multiple clips. Right click and select Fade. A second menu allows both audio and video. Keyframes are inserted and can then be moved with the curser.
Once you've done it to one clip you can select that clip and paste the applied effects to other clips.
In other words to do a lot quickly temporarly add a single, short clip. Apply the Fade. Select that clip because it has only the Fade effect applied. Now cut and past that to all the clips you want to fade in and out. Delete the temporary clip.