Im pretty sure simmering your cider and adding a whole apple to it wont hurt really anything. But neither are necssary.
Couple of thoughts...
Heating cider to 160 deg F before fermentation will certainly kill any wild yeast and bacteria. It also likely drove off some of your apple flavors. As well if fresh pressed could set the pectin, making it a lot harder to clarify.
I cant really think of anything a single whole apple will do other than float around and if not kept submerged or wet by shaking or swirling the ferment could give any "nasties" to cling to and create mold. Keep an eye on it.