Well, to make the arrows work a replaced the "on tap gesture" condition with the "is touching", and it work just fine.
Now for the double tap scale rate, it's not working at first because every tick you set the scale of the object to the size set on the slide bar, so when the object is resized due the double tap it always go back to the slide bar size.
However when I disabled the every tick code line it still didn't work because for some reason the double tap is not changing the value of your variable.
Sorry for not being able to help any further