I would not give the sick hen ivermectin. It is just going to make her worse unless the cause is roundworm infection, which I doubt it is, since those are not normal symptoms of that illness. Giving her a poison is just going make things worse. If it were me, I would bring her to see a vet -...
I am not sure why you would put them on corid when they were vaccinated for coccidia. They might have thiamin deficiency, especially if you are also feeding them medicated chick feed - http://www.poultrydvm.com/condition/thiamin-deficiency
I don't believe it needs to be PVC coated to be effective. However, here is a link to an infographic which might help explain better - http://www.poultrydvm.com/featured-infographic/tips-for-protecting-poultry-from-predators
Why can't you keep them all? There is no magic ratio for number of hens per rooster. It depends on personalities. They might get along fine. Or an alternative is to start a separate bachelor flock - http://www.poultrydvm.com/featured-infographic/tips-for-keeping-bachelor-flocks
Here is an infographic which provides some tips which answers your question - http://www.poultrydvm.com/featured-infographic/caring-for-newly-adopted-chicken
Mareks would take longer then 48 hours to cause the bird to die, plus most chicks from hatcheries are vaccinated against it. Have you tried using the chicken symptom checker tool? http://www.poultrydvm.com/views/symptoms.php
You really ought to bring her to see a vet. Dogs carry bacteria in their mouth which is deadly to chickens. If she doesn't die from the wound, she'll die from that. Here is a case story about it - http://www.poultrydvm.com/cases/goldie
Each rooster has their own personality. It is just like each person is different. However, I have found most roosters to traditionally be very good at taking care of hens, no matter how they were raised. It is their instinct. However, if you super concerned, why not simply adopt an older rooster...
Why not just temporarily separate him from the hens for a little bit? Roosters are typically most aggressive right after they first develop their hormones, then they calm down.
Why are you having trouble keeping them alive? Here is good info on how to properly predator proof - http://www.poultrydvm.com/featured-infographic/tips-for-protecting-poultry-from-predators
You shouldn't give Amprolium and B vitamins at the same time. Here is a reference to info on this - http://www.poultrydvm.com/condition/coccidiosis It doesn't sound like coccidiosis to me though. Have you tried bringing them to see a vet?
You might benefit from using the poultry predator identifier tool - http://www.poultrydvm.com/predator-identifier.php as it could have been many different animals...
Yay! How fun! Roosters are alot of fun! I LOVE my roosters. They make the experience of having hens so much more fullfilling because of all the cute things they do. I wouldn't worry too much about introducing him. His job is to protect the hens, he will mate with them, but it won't hurt them.