Sick chick! Please help!

[Wyorp Rock]

She is having a hard time breathing. She doesn’t open mouth breath all the time.

She is sneezing or what seems to be a sneeze. She shakes her head and like like blows out of her nose.

Please look inside her beak to see if she's got food or something stuck in there. Look in her ears to make sure there's no lice/mites or signs of infection.

When she is standing she’s falling asleep a lot. Not lethargic though.

To me, she doesn't look lethargic or sleepy.

She is preening and is losing like ‘dandruff’. She’s pecking at her legs.

Dandruff is keratin sheaths, preening is normal. Some chicks are a bit "itchy" when going through juvenile molt. Pecking at her legs - looks like she's trying to remove debris from her toes. Clean up her feet and keep her on clean dry bedding.

She is eating and drinking and pooping
I have put Corid in her water. I have rubbed Vetrx on her beak and feathers.

How much Corid are you giving?
How warm are you keeping her?

I do agree, she's snicking quite often, but I would look to make sure there's no obstruction inside the beak, the VetRx isn't going to hurt, see that she's got adequate ventilation. Really check her well, just to make sure mites are not an issue as for the scratching.
It's possible she may have respiratory illness. If you feel she must be treated, then you can order Tylosin online and treat her. Likely if that was the issue, then her sisters would be showing symptoms by now.
Photos of the poop would be good.