[KevinH]

For your 6. that is json, not css or xhtml. Nor is it xml. Exactly what is a "custom element" and if this is metadata, then does it even belong inside the opf?

For your 7, just drag to highlight. Shift Double-click has already been defined. That is the trade off made when that feature was added. (Is that what you meant by Maj Double Click?).