the problem you're talking about is creating a checkbox using js / json. if you look at what is actually stored in the JCR, you will see that the value against the checkbox is actually not a boolean, but a string. which can be true / flase or on / off if the item is inside the checkbox group.
In your case, to make it automatically checked in your dialog.xml file, you need to use checked = "true".
Kelly b
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