Did the vet say this was DKA or that Marshmallow was showing ketones? With DKA, electrolytes are out of balance.
Also, did you skip the last 3 shots? If that's the case, then this is a lesson learned. With a cat prone to ketones, you want to do everything you can to avoid skipping even one shot. The recipe for ketones is not enough insulin, not eating, and an infection/inflammation. You had no control over the UTI which, like the vet said, probably put this in motion.
Your job is to see if the vet will teach you how to administer fluids. That helps to flush ketones out. You need to test for ketones as often as you can. The earlier you catch ketones developing, the easier it is to manage. You also need to maximize insulin. Don't think because Marsh is seeing good numbers that ketones won't occur. For a cat that's prone to ketones, where the numbers are may not make a difference. Any time you see a change in behavior, especially lethargy, your antenna should go way up.
NovaMax makes a meter that will test for ketones and BG. They have a promotion where you can get the meter for free. Abbott makes the Precision Xtra which also tests for ketones. The down side of these meters is that the strips for testing ketones are expensive. Using Ketostix from the pharmacy is much cheaper.
Keep us posted on how Marshmallow is doing.