If your melatonin comes in plastic capsules, like mine, you can twist them apart, discard some amount of the contents (I tend to roughly-fill the smaller of the two caps and throw the rest) and optionally put them back together, without too much hassle.

Nope. That's moderate Civilizational Incompetence; science knows well that 1mg is often way too much for a first dose, but shops have presumably found that the average customer thinks "More melatonin is better" and that informed customers are too scarce to market to. You can get correctly dosed melatonin on the Internet, as with any other niche market.