Crop or for that matter gizzard impaction is something that just happens to affect certain chickens, apart from keeping young birds off long grass I don't personally believe there is a huge amount you can do about it prevention wise. With regards to a cure for crop impaction dipping the birds beak in olive oil to make it drink the oil followed by a crop massage has proved successful for me, in cases of gizzard impaction death is inevitable. Watery stools and lack of appetite are certain symptoms along with the sick chicken posture. At the risk of being harsh I believe that the vast majority of vets are no better than snake oil merchants when it comes to chickens, they really don't have a clue as in agricultural terms chickens are expendable, if you are a farmer and you have a sick chicken you just neck it, so they have no real experience of treating chicken disorders. Almost every post I see on this forum where someone has engaged the services of a vet seems to involve a prescription of antibiotics along with a vague (and unlikely) diagnosis with a hope that the former will prove successful, often concluding with the sick chicken being pts. I am very sorry for your loss chickenfan you did everything that you could, I don't think that you did anything wrong, my first experience of chicken keeping was losing a bird after only three days to a compacted gizzard from long grass, I don't even know if it was from my garden or where she had come from, effectively the first thing I learned about chickens was how to dispatch them.