Update:
Having given myself ownership and permissions to ~/.local/share/Trash[/*],
I am now able to trash items that are in my system / partition (including items that
are in /home/scott that are NOT in the bind-mounted personal data directories)
with no complaint from Caja.
scott@ASUS-PRIME-B350M-A-CSM:~$ ls -lha .local/share
drwxrwx--- 4 scott scott 4.0K Mar 16 2019 Trash
scott@ASUS-PRIME-B350M-A-CSM:~$ ls -lha .local/share/Trash
total 16K
drwxrwx--- 4 scott scott 4.0K Mar 16 2019 .
drwxr-xr-x 19 scott scott 4.0K Oct 29 23:50 ..
drwxrwx--- 2 scott scott 4.0K Mar 16 2019 files
drwxrwx--- 2 scott scott 4.0K Mar 16 2019 info
However, I am still not able to trash files in my bind-mounted directories.
In looking at the terminal output for the trash directory for /data (see above),
I noted that although I had ownership of the directory, only the owner had
permissions for "Create and delete".
I fixed that...
scott@ASUS-PRIME-B350M-A-CSM:~$ ls -lha /data/.Trash-1000
total 4.0M
drwxrwx--- 5 scott scott 4.0K Feb 28 2014 .
drwxrwxr-x 10 scott scott 4.0K Oct 28 20:15 ..
drwxrwx--- 2 scott scott 16K Feb 28 2014 expunged
drwxrwx--- 176 scott scott 1.8M Oct 26 20:48 files
drwxrwx--- 2 scott scott 2.3M Oct 26 20:48 info
...but still had the problem...
AT LAST, I believe I know what the problem is!
I opened the same directory twice, first as:
/home/scott/Pictures/HighestNumbered
then as:
/data/scott/Photos/HighestNumbered
I duplicated a photo.
Caja was unable to trash:
/home/scott/Pictures/HighestNumbered/TGED6979 (copy).JPG
but WAS able to trash:
/data/scott/Photos/HighestNumbered/TGED6979 (copy).JPG
...so Caja has a problem trashing items from areas that are
bind-mounted.
Caja has no problem displaying/copying/moving items, whether their
location is the directory in the /data partition, or in the equivalent
bind-mounted directory within my home directory,
but it CANNOT trash them, if I point it at the home directory equivalent.
Relevant fstab entries:
# <file system> <mount point> <type> <options> <dump> <pass>
UUID=326c8c32-18d2-4322-91d0-ebc73cf9ed0e / ext4 errors=remount-ro,noatime 0 1
UUID=bc0d2436-d89c-4a70-abd0-bb39f092993f /data ext4 defaults,noatime 0 2
...
/data/scott/Desktop /home/scott/Desktop none bind,x-gvfs-hide
/data/scott/Downloads /home/scott/Downloads none bind,x-gvfs-hide
/data/scott/Templates /home/scott/Templates none bind,x-gvfs-hide
/data/scott/Public /home/scott/Public none bind,x-gvfs-hide
/data/scott/Documents /home/scott/Documents none bind,x-gvfs-hide
/data/scott/Music /home/scott/Music none bind,x-gvfs-hide
/data/scott/Photos /home/scott/Pictures none bind,x-gvfs-hide
/data/scott/Videos /home/scott/Videos none bind,x-gvfs-hide
('x-gvfs-hide' is necessary to prevent file managers from displaying each directory as an ejectable device
in Places sidebars and in the /home/<user>/ directory.)