Gapeworm or resp. illness?

Hey folks, I need some help. I believe my 6 yr old Asian Cochin has gapeworm. She keeps straining her neck n makes these barking type noises. I called all the feed stores and it seems no one locally carries antibiotics for hens anymore. I gave her a bb sized dollop of safe guard (horse wormer I was told was safe for hens) 2 days ago and I gave her another tiny bit last night. She is still eating and drinking and wondering around outside like normal. I have added apple cider vinegar to their waters and I have been sprinkling algae powder that has enzymes and probiotics besides vitamins and minerals on their pellets. I have raked their outdoor area and cleaned all the shavings off the floor of the coop. I bought some food grade DE that I am going to put down when I go out to strip their boxes and put down fresh shavings. No other hens are showing signs of this ailment.
I am also going to go put out some crumble with fresh garlic and break off some of my horses red salt lick to add to the water.
Any other suggestions. This is my fav hen n I hate seeing her like this.
I have been on youtube trying to see how to swab her throat with a qtip but not finding anything.
I saw a post on here that gapeworm won't show on a qtip, but other places say a red stringy worm will be on the qtip if that's what it is.
There's sooo much conflicting info out there.
Should I try swabbing her throat?
Also, I was told I could get fish antibiotics and that would work. The feed store here has Amoxicillin, cephalexin and ampicillin.
Are any of these better than another for worms in hens?

@casportpony has great threads and information on worms. I have no experience with this sorry.