As you probably know, most Upper Respiratory Infection's (URI) or the cat flu are going to be secondary to viruses with Herpes or Calici viruses being the two most common causes of this condition in a cat.
Secondary bacterial infections can develop (the nasal and/or eye discharge can be green or yellowish in color) and when this happens, antibiotics are usually pretty effective at helping to get these infections under control. I personally prefer Clavamox to Amoxicillin since there's so much resistance to it these days but if that's all you have, then this is what I'd give.
If the underlying virus is Calici, then ulcers can develop on the nose, tongue and surfaces of the mouth which are very uncomfortable/painful. These cats will often drool or show disinterest in food if severe enough. This may be why she isn't eating as much (she's too painful) although she could also be running a mild fever.
These episodes typically last anywhere from 10-14 days just to give you an idea of the time line.
As you no doubt know, supportive care will be cricital for Bandit, especially given her age. Towards that end, the following would be my suggestions:
1. Give the fluids you have SQ since it's going to be quite difficult to get an IV catheter into her. I'd give approximately 100 cc's once or twice a day given her weight. If the fluids aren't absorbed and have pooled down her arm or the side of her body, then I'd not give any more until it is.
2. I understand your use of Benadryl (to help dry up secretions) but it hasn't proven to be that terribly effective in these patients. It also has the potential downside of causing sedation which is not a symptom we want. So, I might rethink use of this drug.
3. Use warm water to clean the eyes and nose of any discharge that builds up which you're probably already doing. Over the counter artificial tears can be used to flush out the eyes if necessary.
4. Nasal decongestant drops can be helpful although many cats don’t like them very much as you can imagine. The medicated ones (a-c below) should only be used for three days in a row, with one drop in each nostril; otherwise what’s known as a rebound effect may occur. This is one reason why I prefer"d":
a. Pediatric otrivin=0.05% xylometazoline
b. “Little Noses" Decongestant Nose Drops with phenylephrine hydrochloride
c. Oxymetazoline which is the primary ingredient in Afrin or Zicam
d. "Little Noses" Saline Spray/Drops non medicated which can be used more than three days in a row.
4. You can place her in a bathroom with hot water running so that the steam can loosen up secretions which is somewhat wasteful of water; a better solution would be use of a humidifier to moisten the air she breaths.
5. L-lysine can help with any eye issues if they're present.. Herpes viruses can settle in the eyes and cause l issues there This supplement would be given daily (250 mg) The feline product is available on the internet as either a chewable treat or a paste; the human version comes in a capsule and the contents can be sprinkled on wet food (if the patient is eating) and given this way as another option.
6. Force feeding is sometimes necessary in these cats if they aren’t eating. You can mix canned food (or human baby food but avoid onion and garlic formulations) with a lot of water in a blender and use an eyedropper or syringe to squirt some of this liquid into the side of the mouth where there's a gap. This will be especially important since she's older with fewer reserves than a younger cat would have.
7. Probiotics such as Forti Flora have been shown to help many of these cats with URI's; probiotics aren’t just useful for gastrointestinal issues. This particular product is available as a powder which is sprinkled on the food everyday (or mixed with #6) and can be purchased at many pet/grain stores or online.
Hopefully, the intensive treatment protocols as outlined above will be sufficient to get her past this. Deb