The timing of watering sounds ok just wondering if it might be too much for a young, newly planted tree. Try keeping the sprinkler off of it for a week or two and see what happens. The amount of watering needed can vary depending on your soil type and how hot the weather is. Also the siting of the tree. If it is in full sun or less than six hours sun. Most people do tend to over water than underwater so I think that may be the problem. It needs time to develop a proper root system.
The spray you used sounds appropriate for the job as long as it was applied as per instructions.
Thought you might be interested in an extract from a study done on the main constituent in Sevin and its affect on the other wildlife in your garden.
Quote[We do know that carbaryl is quite toxic to honey bees, certain beneficial insects such as lady beetles, and parasitic wasps and bees, certain species of aquatic insects, and some forms of shellfish such as shrimp and crabs. Care must be taken when using carbaryl in areas where these organisms exist.]