Most had not. CPIM was intended to address common elements. When you introduce multiprotocol messaging, things get complicated. Server 2 could use SIP protocol with Client 2. This led to a presentation by Derek Atkins, who gave an overview of the outcome of several offline discussions through the week between some of the active participants in the various proposals on the table.
Signatures are necessary to tell that the message has been changed.
Maybe the data header should just be a number, to avoid people trying to stick funny date formats. Proposing an approach to the problem, but details don't have to be locked down as part of the philosophical approach. Most intermediaries would not show you their trust model. Transport might include a set of transport headers that might even duplicate information that's part of this. Given that the message can be reformatted during its route at the end it is changed back to the the format where you can verify the signature.