So I'm editing pitch and modulation wheel events in the PRV controller panes. 
I can select a group of controller events using the right-click selection marquee. 
If I click and drag on the selected events, I can move them around as a unit. I get that.
 
However, what I really want to do is *scale* the events. I tried Shift-click-drag and Ctrl-click-drag, and although the cursor changed to a different icon from the "move" icon, nothing seems to happen to the events.
 
I expected SHIFT- to lock the y-axis (value) so that I could click-drag the events back and forth along the timeline, without changing their values.
 
I expected CTRL- to allow me to scale the event values whilst locking their time and 0-base. (in other words, reduce the strength of a pitch-bend or modulation curve)
 
Am I missing something?

AFAIK, I don't think you are missing anything.  The PRV controllers pane doesn't support all of the usual Smart tools modifiers found in the Track View.  Sonar 2015 did add a "scale" velocity tool using Ctrl-SmartTool, but it does not appear to work for CC events.
 
The only method I know for "scaling" CC events in the PRV is the Process > Find/Change... dialog.
 
For example you might have a CC 1 curve with a range of 20 to 100 and you want to flatten the curve so it has a range from 20 to 75.
 
1. Select all CC 1 controllers in the controller lane.
2. Run Process > Find/Change.
3. In the "Search" dialog, clear the dialog by clicking None and click the Control checkbox.  Enter 1 for the Min and Max "Number" fields.  Enter 20 for the Min and 100 for the Max "Value" fields.  Click OK.
4. In the "Replace" dialog, change the Max "Value" field to 75 and click OK.  The curve will be proportionally flattened.
 
Hope this helps.

What happened to this thread ( i can only find the one that Google cached now)?
 
http://forum.cakewalk.com/PRV-Please-return-quotShow-controller-event-handlesquot-m3178831.aspx
 
I think this is related, because controller handles were what allowed you to grab and scale them.

Hey Brundle - scook deleted that thread because I realized I had misunderstood how controllers work in SONAR_2015.
 
I actually prefer how controller events are adjusted, now. You can still select groups of controller events, or edit them individually (it's actually easier now, I think.)
 
However, I do believe there are bugs or un-finished features in the Controller Pane Editing toolset, to do with scaling and selecting. I plan to make some gif-movies of these things later today, and post them as question/bug threads.