Hi Daté,
I have run into the same problems as you. XMP Panels not showing up, staying empty can be the result of a series of errors.
SDK version versus CS version, errors in the code and 'sticky' panel preferences will all result in empty panels.
I agree that Adobe made Custom Panel creation in Flex/Flashbuilder overcomplicated.
And the documentation is not always complete. Up to date information is scarce.
Setting up your development environment correctly, prior to building is tedious to say the least.
It took me 2 weeks to figure out that FB 4.7 is not compatible with Extension Builder 2...
I found the 'easiest' way to create Custom XMP Panels is Flashbuilder 4.6 with extension builder.
About InDesign plugins: I don't know if plugins that specific exist. E-Spec does a lot in this area.
Nevertheless XMP data is accessible through scripting. The thumbnail is a base64 piece of code between the <xmpGImg:image> tags.
Regards,
Paul