Shielding itself isn't the issue, it's that while you're holding it to protect yourself, going near anyone pushes them with you, and this would just be an excuse to prop push people off edges - "Oh, I was using the minecart as a shield but he got in the way" for example.
Other than that, a clarification of the RDM rule is fine, and giving people time to respond to warnings is kind of detrimental to detectives forcing people to test as they are wasting more time on one person, but not an issue that would outweigh the benefits the change would bring.