If my Charmander flung a Toxic Orb to a Squirtle, then the Squirtle would be poisoned right ?
Yes. The effect of the item kicks in, and since the effect of the item is poison poor Squirtle here gets poisoned. (Source: Here)
What happens if the Squirtle used Fling and had no item ? Would it "fling" back the Toxic Orb to Charmander ?
No, since the move fails if the Pokemon has no item. (Source: Above Link)