With macOS 10.14, it appears not to be possible to disable local snapshots.
You can remove local snapshots using a method documented in Apple's About Time Machine local snapshots support document:
How local snapshots use storage space
You don't need to think about how much storage space local snapshots are using, because they don't use space needed ...
The command you are using adds any significant amount of free space immediately after the APFS container partition. By adding the values posted in your Update 2, you can see the APFS partition container is already at the end of the drive. There is no significant amount of free space to add.
+ 81,926,582,272 B
This should unlock the ability to resize your disk:
tmutil listlocalsnapshots | cut -d. -f4 | xargs -n1 tmutil deletelocalsnapshots
It does the same thing as Neil's answer, except that it copy-pastes the timestamps for you.
The WDC WD100... and the Mercury Electra are devices. They represent the physical drives that are attached to your Mac.
Devices contain one or more volumes. Macintosh HD is a volume on the WDC device. There will be other volumes on that device that aren't displayed.
The Mercury device uses Apple's new APFS format. This uses a structure called a Container ...
As an educated guess after reading around, the likely reason that the High Sierra container could not be resized was that it was not contiguous with the free space.
I have now zeroed-out Mojave and cloned the contents of High Sierra to there, whereupon I can resize that partition to occupy the free space.
For me it looks like you need to resize the container first. So something like
$ diskutil apfs resizeContainer /dev/disk0s3 500g
should help to avoid the error message you get on your second try.
I've got not much knowledge about the diskutil, but as far as I know Apple software overall, this seems to be the step you are missing here.
I booted in Recovery mode, open Terminal and run diskutil unmountDisk /dev/disk0 and after that fsck_apfs -y /dev/disk0
After this all was fine
Maybe for you, the drive name is different, you can find it by running diskutil list