Responsible party for preparing M&V plan

It depends on the individuals involved. It is very hard to make such generalizations but in general the CxA is probably in a better position to do this work since they are already going be doing some of the work required. Be careful however that whoever does it must have a thorough and complete understanding of what needs to be done. M&V is not just some enhancement to the Cx process and requires a somewhat different focus. Who is doing the project's energy model? Typically whomever does the initial modeling would need to be involved in the M&V effort.