By far most of the research reports I read that have a subtitle start the subtitle with an uppercase letter (figure to the left below). Is there any reason why the subtitle should not start with an en dash and a lowercase letter (right figure below)?
There are multiple reasons why the example on the left is better than yours:
First is the issue of capitalization. You're advocating to lowercase the U in Understanding which goes against Title Case.
The next is that the en-dash holds a similar function grammatically to a comma. You would place this punctuation at the end of the top line, not the start of the bottom line.
You're treating it as though the Title + Subtitle combine into one cohesive sentence. If that is the case they need to be a complete sentence including period at the end. You'd also need to make sure that all titles and subtitles can be treated in a similar way.
There's no rule for this.
If you want to keep the dash, what i would do is use a negative indent so the second line tabs right under the first line of text. Then i would use the same indent for the authors below.
But there's different ways to play with this. You can take this further and also indent the main title and logo by the same amount, so the dash will stand out on the left.
Designers have no business being editors unless they have the training to accompany the responsibility. Editors have access to dictionaries, style guides -- AP, Chicago, etc. I'm sure Rand has a corporate style guide as well. Any of these could have an impact on the look of the cover, especially regarding dash usage. As it stands, whatever the outcome may be, not capping the "U" after the dash is just plain silly when all the other words have init caps.