I left town for 6 days, but had been cleaning tank every other day, as I had 2 goldfish and a snail in a 5 gallon tank. On the third day, their "sitter" told me she couldn't even see the fish. On the fourth day, they were removed to a regular fishbowl.

On the 6th day, I cleaned the tank, replaced the fake plants and gravel, and returned the three of them to their tank with some stress coat. Two days later, both goldfish had cloudy eye, and the snail died. One goldfish died the next day.

Goldie is still alive. Treated with Melafix 7 days, did a quarter water change, and replaced carbon filter 4 days ago. (Also removed all of the "obstacles" so she wouldn't hurt herself). Her eyes seemed to get worse (the "cloudy" seemed to poof out for 5 days of the treatment), now they seem *almost* cloud-free, but she is still blind, foraging the bottom for her food.

What else should I do for her?

Is she a single-tailed goldfish (like a common or a comet), or is she a fancy (like a fantail or oranda)?

Single-tailed goldies can grow up to 12 inches long and are pond fish really. A single fish would require at least 200 litres to itself.
Fancies don't grow quite as big and are less active, so you can get away with a slightly smaller tank. A single fancy would need at least 100 litres, though more would be better.

Either way, your goldie needs a bigger home asap. In such a tiny tank, your problem will be down to poor water quality, and there is no way to keep the water clean in such a small volume of water.

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Goldie appears to have made a complete recovery -- she's one tough fish!