Can't remount /system in rw
The syntax of mount command usually requires you specify the target:

mount -o remount,rw /system /system


This output could be useful for us to better understand your problem:

cat /proc/mounts


As a last resort, as you have root you can try saving raw image of system, mount it on your box and push the app there, then flash it back on your device. To save raw system image:

dd if=&lt;block device mounted on /system&gt; of=&lt;output file&gt;


If you go that way make sure you keep the original system image in case something goes wrong.