Regarding the trace ketones and feeding:
If your cat is underweight, to prevent DKA it's important that you feed AS MUCH as she will eat. This is going to raise the blood sugar due to the extra calories available in her system. That is ok, you use the insulin to account for that increase. The extra food + the extra insulin will clear up any ketones as well as help her gain weight. Although some calorie/feeding restriction is needed with obese diabetic cats, underweight cats should not be restricted. Underweight cats who don't take in enough calories tend to become "brittle" diabetics, that is, their blood sugar can swing from high to low and back again and never get evened out. The more calories you can get in, the more stable the blood sugars will get, although you'll probably need a higher dose.
In regards to the type of insulin you are using, Novolin peaks at about 6-8 hours or less in cats, and has a total duration of about 8-10 hours, sometimes 12. So really the majority of the insulin is used in the first 8 hours or so. What ends up happening is that for that final 4 hours until the next dose, her system is without insulin. With no insulin, a body cannot use the sugar in the blood for energy, and ends up burning fat instead. Most cells can use ketones for energy for a short while, but these will build up in the blood and cause DKA. Novolin is not a good insulin when a cat is prone to ketones, as it doesn't provide a full 12 hours of coverage. Lantus and levemir peak in 10-12 hours but have a residual effect for up to 24 hours or more, so doses overlap. This means the body is never without insulin - it's called a basal dose. Having a basal dose is very important to preventing DKA in ketone prone animals and people. In order to have this with novolin, you'd have to shoot every 8 hours. You may want to discuss switching insulin types with your Vet.
Also, if your cat is currently eating, that is a good sign. Continue to test ketones daily, as they can build up and become a problem very fast (less than a day). If your cat stops eating again, or if ketones are higher than "trace" you need to see the vet as this is somewhat of an emergency.
Edit: The novolin I'm referring to above is Novolin N