I think the hierarchy could be made clearer by removing the indentation from the blue box of events instead of increasing sub-event indentation. This way a line is created on the border that can quickly be scanned by the eyes, revealing instantly the hierarchical blocks.
Something like this:
*click images to view zoom
(left: current evensheet, right: without indents)
Also removing the lateral horizontal lines could help making it a bit more clean. The horizontal lines just add some unnecessary complexity while the indentation of sub-events already accomplishes the goal of indicating nesting.