Hello, I'm Dr. Deb and will do my best to help you today although I'm sorry that you've had to wait for a response.
I'm also sorry that Scratchy has been diagnosed with an URI. While most cats will survive this particular condition, I do worry about the older cats like her since she doesn't have the reserves of her younger counterparts.
Adding pedialyte to the baby food you've been feeding her is a good thing to do. The electrolytes which it contains can be quite helpful since she's not eating on her own.
Just to give you an idea of her nutritional requirements: A 4 lb cat would need about 100 calories or a little more as basic maintenance in a 24 hour period. I'd probably try to get a bit more than that into her.
Her fluid requirements will also be about 100 mls or cc but will more difficult to calculate since there's likely to be moisture in the baby food.
I might ask your vet about an appetite stimulant such as Mirtazapine or Cyproheptadine which may help in getting her to eat again.
I'd also consider that the Clavamox she was prescribed may be affecting her appetite since anorexia can be seen as a side effect with this drug.
Nasal decongestant drops may be helpful in clearing the nasal passages 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. Afrin (oxymetazoline)
d. "Little Noses" Saline Spray/Drops non medicated which can be used more than three days in a row.
As I mentioned above, most cats will recover just fine from a URI, especially if supportive care is provided for them...keeping them hydrated and receiving enough nutrition.
I hope this helps and that she's feeling better soon. Again, my apologies for the delayed reply. Deb