re: Man-rating an existing rocket
"Existing rockets" worked just fine for the manned space program up until Apollo. Most of them were missiles with the serial numbers filed off.
The key is a decent escape system.
NASA deliberately tuned the requirements to need a new launch system. NASA is a development organization, and if there are two ways to do something, NASA will do the one that needs the most new development.
Also, there's no reason the Orion capsule has to be that large for orbital operations. There are other ways to do lunar and deep space missions.