The vitamin C you get in powder form - it is given by weight and will state how much you should give on the packaging. There's one that is especially for cats and dogs (called Halo X-tra C) I mix it under the food, same with the Solid Gold Seameal.
For the Transfer Factor I would suggest you google it - it's a combination of various herbs which support the immune system, combined with Colostrum. The problem is, this does come in capsule form. As the lump is in her throat it will be difficult to give it in the capsule form - so I would suggest breaking the capsule open and giving it to her at feeding times. All of the things I give Lucky (exception the injections) are in powder or capsule form. All capsules can be opened up and the powder put into the food, that is, if she is still eating. Just make sure that your cat continues to eat. I know very little about cats, however I would suggest mixing these supplements under something that has a strong taste, such as tuna.
Unfortunately there is no guaranteed cure out there, and I am just as much in the dark sometimes as you probably are. It is a very, very difficult thing to deal with. Just remember to keep being positive.
I know this might sound off the wall, however I have worked with an animal communicator to find out what exactly Lucky needs and have been able to adjust what I give her. It is nonetheless trial and error and there are days where I am angry and frustrated but for the most part both Lucky and I have excellent days.