Oh silly me You'd think the big Australian flag might have clued me into the fact that you were from Australia.
I have a few friends down there and they've been complaining about the cold, so it's nice that your area is mild.
Two things would make me highly suspect a calcium deficiency and egg binding. The droopy wings, sluggishness, the lack of an egg, and her gulping down grit. Birds will sometimes do this in an effort to regulate their calcium - bring it up.
It wouldn't hurt her to give her a calcium/vitamin D boost. Do you have "Tums" brand anti-acid tablets there? You can give them a quick emergency boost of calcium by crushing one of those into their food. Organic apple cider vinegar or plain yogurt (fortified with vitamin D) or a little bit of milk (also fortified with vitamin d) would boost the vitamin D content, which is necessary for calcium absorbtion. Also, it's important not to dilute the calcium impact of laying pellets with too many grains. Grains, high in phosphorus, can mess with the cal/phos balance of laying pellets causing calcium to not be absorbed enough for laying and, in fact, actually with drawn from the bird's bodies and bones if phosphorus levels are too high. I'd give her no more than 10% of anything other than her pellets (not including her free range experience.)
So I'd concentrate on this until there's more information about something else.
On egg binding, feel her abdomen very gently for swelling, any lumps, etc. You can actually even check for a broken egg by coating a finger in KY jelly or vaseline (I prefer the former) and gently examining the enterior of the cloaca for shell fragments. Watching her droppings can also tell you if there's any yolk in the droppings.
During the incidents with the parasites, she might have had her blood levels of nutrients go down and just now is showing it - including the calcium. I'd try to boost it up, make sure she's hydrated and whatnot, and see what happens over night.