I can't watch the video right now but you saying that she is raspy and that others are starting to show symptoms makes me think it is respiratory. I don't know what you can do if it's viral, but if it's bacterial you can give them tylan 50. It's an injectable medicine they sell at Tractor supply, if one is near you. It should be available at most feed stores though. Let me know if you want help with dosing/injecting.
If you haven't already, I would separate the sick ones from the healthy ones. They have probably all been exposed but I would still try anyway.
I don't have any experience with gape worms but if you suspect worms, then you can get a fecal done by most vets, even if they don't treat chickens. Fecals are done the same, regardless of the animal and it only takes minutes to do. I'm not sure if gape worms show up in fecals, but most worms, even the ones that don't live in the gut, will pass eggs through the gut. Worth a shot and I've never seen a fecal cost more than $20 (usually $10-15).
If any of them do pass, consider sending the body off for a necropsy. If you do a search on here, there will be information on how to send them and some resources for who will perform them in your area. In the meantime, the important thing to know is that if you are going to send them, put them in the refrigerator NOT freezer until you are ready to send them.
Hope this helps!