I dont think you should confuse an overall description of the ground taken from a dictionary with what it might mean in a cricketing context.
With no distinction made in the cricketing laws I think we can take as given that the grass should be classed as part of the ground in all cricketing matters
Eh? Why on earth would we do that? That's daft and illogical. By definition, The grass is not the ground, so why would we pretend that it is. There is no argument in favour of re-defining the ground as being both the solid surface of the earth AND any crop or plant growing on top of it, either through deductive reasoning or matter of precedent.
People have been playing cricket for 100s of years, and the grass has never been considered part of the ground. If there was a particularly long 3 foot strand of grass sticking up in the air, and in taking a slip catch 2 foot off the ground, the ball brushed it on the way to your hand, are you honestly claiming that the ball "hit the ground" an thus you would tell the batsmen he was not out? People would laugh at you.