How to move a folder to another path and to move back to same tree merging its contents on Unix?
One approach is not to move directories. Let them exist all the time, move their content back and forth:

mv -f /sdcard/WhatsApp/Media/* /sdcard/wppmediabkp/
# and later
mv -f /sdcard/wppmediabkp/* /sdcard/WhatsApp/Media/


Notes:


* will not match hidden files/directories;
with too many objects you may hit "argument list too long" error.


In case any of this is an issue, the solution should use something like find … -exec mv …. Either way move what is inside the directory, not the directory itself.