If she were mine I would tube fluids to her at 8-12 ml per pound, repeat in 60-90 minutes, then again in 2-4 hours. Never force feed a bird that has not been drinking. Hydration status must be corrected first.
Watch this:
https://lafeber.com/vet/tube-feeding-birds/
Look at this : 8-12ml per pound of bird. How much do you think she weighs?
If she's 5 lbs, you can try giving her 40-60 ml fluids at a time, then repeat that an hour to an hour and a half later.
If you happen to have nutri drench on hand, add some of that, otherwise add plain sugar to the water (it should taste sweet to you) - that way she can get fluids and energy at the same time.
If you are still giving Corid, do not give any supplements containing B vitamins at the same time. Corid works by starving the cocci of vitamin B - if you add more vitamins to the diet, you are decreasing the effectiveness of the drug.
If you are not comfortable tube feeding (and I wouldn't be), drop in whatever you are trying to give on the side of the beak, very slowly - not from the front, as that might go down the wrong pipe.
Now, cocci is less likely with an older bird - but you have had her for just 9 months and any change of environment can expose them to new types of cocci.
If they have never had warm , moist conditions before, it is possible that those new types of cocci are only becoming a problem for them now. They can also encounter new types of cocci that have hitched a ride on your shoes if you have been to a place with chickens lately.
If you give the dose for a severe infestation (the 2 tsp per gallon) and it is in fact cocci that is bothering your hen, she should feel and act better fairly quickly, in maybe two days. Her comb and wattles should look brighter and most likely you'll notice that in the hens that haven't been acting sick, too.
If Corid isn't making a difference I would go for a wormer.
Unfortunately there are many other things that could be wrong with your hen that you can do nothing about.
I am keeping my fingers crossed for you!