acrobat xi -- can someone recommend a good, in-depth, tutorial/reference for creating custom dialogs

To echo Karl, what you're asking for doesn't exist. I've been writing Acrobat JavaScript dialogs since at least Acrobat 7 and I'm still learning new techniques. Still, the best place to get started is pdfscripting.com. The time it saves you is worth the subscription.

Unfortunately, there is no such thing - or at least I am not aware of anything that comes even close to what you are asking. Keep in mind that custom dialogs are among the most complex things you can do with Acrobat's JavaScript.

In order to create custom dialogs, you need a good understanding of JavaScript data structures. There is no way around that. You are basically creating an object when you create a dialog. You need to know how that is done in JavaScript in order to understand how the dialog object needs to be created.

You can find some tutorials on pdfscripting.com, but you will need a subscription to view them: Video Tutorials

Here is what I would suggest: Get familiar enough with JavaScript so that the examples in the Acrobat SDK will not scare you away. Then try to understand the documentation for app.execDialog() - that includes understanding all the examples.  You can find the documentation here: Acrobat DC SDK Documentation - app.execDialog()

If you need help to get started with Acrobat's JavaScript, take a look here: Beginning JavaScript for Adobe Acrobat