Are there any tutorials or documentation on a few of the less common features below?  I normally use a Holy Grail or Visual Workflow as per tutorials, but don't have a clue what these other things do:

Tools Tab - Individual Transitions
Create Snapshot button

How do you target just one portion of the sequence to deflicker?  I see there is some ability to drag and select in the preview pane.  What are the reasons to do that / uses?  

Thanks!

Tools Tab - Individual Transitions

In 99,9% of all cases the Auto Transition is what you need. Those individual transitions can be used, if it doesn't or if you want to apply a special transition so single values or only a part of the sequence. Basically those are the old features, before the auto transition existed. I don't use them anymore.
 

Create Snapshot button

Allows you to create a snapshot of the current edits. Basically this saves the XMP data of the full sequence. It allows you to experiement with alternate edits and always come back to that snapshot. You can create as many snapshots as you want. 

How do you target just one portion of the sequence to deflicker? 

Just select the part in the table, then apply deflicker.  
 

I see there is some ability to drag and select in the preview pane.  What are the reasons to do that / uses?  

This sets a reference area for deflickering. Only the part inside the rectangle will be used to determine the flicker.

Many of those special features are explained in my EBook: http://lrtimelapse.com/shop/ebook
I'd really recommend it, if you want to learn the more advanced features of LRT.