Thanks for trying to condense this down. I too would like to have a good discussion on this.

I think your assessment of "View A", "This standard should dictate internal design decisions regarding entity body manipulation", is inaccurate. As I've stated, it forces no internal design decision. It provides a standardized interface for interacting with the underlying representation. The underlying representation may not actually be capable of being modified, seeked, etc, but those behaviors are exposed through methods on the interface being proposed.

I think "View A" is: This standard should dictate the API of a message body using an Interface that describes the capabilities of the message including reading, writing, and seeking at the potential cost of some performance (with the baseline being things like native PHP strings).