<< Can bar settings only control the beginning of each bar? >>
Yes. Bar settings affect the whole bar and start at the beginning.

<< How do I want bar settings to control the rear D (red circle position)? >>

I assume you mean the Change Volume Settings column that does the following:

....... nothing
Mute
Back to normal
Change by
Fade (per bar)
Louder (per bar)

If that's correct, then open the Audio Editor and all of these actions can be done there using the new Volume Automation Feature.

I've attached a screen shot of automation doing the following actions from left to right across the screen:

Increase volume; decrease volume; mute volume; faded out; fade in; return to normal.

You can zoom in and easily edit any region. Note that I edited all of the above actions across 2 beats of a bar like you requested to do in your post.

Charlie

Attached Files (Click to download or enlarge) (Only available when you are logged in)
Volume automation.png (50.06 KB, 140 downloads)
Volume automation in the Audio Editor.
BIAB 2025:RB 2025, Latest builds: Dell Optiplex 7040 Desktop; Windows-10-64 bit, Intel Core i7-6700 3.4GHz CPU and 16 GB Ram Memory.