I agree with your analysis of this situation and your decisions for handling it. Even though this is printed in index format it serves the purpose of a table of contents and it is printed at the usual location for contents. Possibly the intent is to serve both purposes--to show what's in the book in a way that also allows for quick location to a specific poem. Follow the braille formatting for an index as you have described and also do not insert that blank line for alphabetical divisions.
I recommend that you treat this as contents and keep it as preliminary pages and you would in a conventional table of contents. If the PRINT heading is "Table of Contents," absolutely keep it in braille.
And just to clarify, Formats is not limited to textbooks. The official title Braille Formats: Principles, etc. never mentions textbooks. These format guidelines are for use in any print material.