The calendar shift seems like an interesting suggestion. It would certainly be more interesting to see the mains reflect the budgetary changes than to wait throughout the year for the supplementals to update what's actually happening.
The reports on plans and priorities estimate departmental spending and program area spending for the current year and the next two years into the future, as I'm sure you know. However, they often only report the present year, so it's difficult sometimes to get an idea of how program areas, for instance, are changing over time.
In terms of reconciling the RPPs with the main estimates, you have a variety of other issues as well when you start to incorporate the cash versus accrual problem, as I'm sure you know, but I think the RPP documents are some of the more accessible ones in terms of looking at how government spending is changing into the future. They can be quite useful if you want to see what departments are planning to spend two or three years out, not only at the department level but also at the sub-level, so I'd argue that they are a good foundation for reporting on how government expenditures are changing and also on why they're changing over time.