I know that in PRV I can select a group of notes and then change their duration en masse by dragging the left-right arrow on the right side of a selected note. However, this tool seems pretty limited, as all it really can do is change the durations in a purely linear fashion. Is it possible to edit duration on a percentage basis, the way I can with the Velocity dialog? ?Even more useful would be to automatically adjust the durations in the selected passage so that every note takes up the entire duration before the next note. "Fully legato" so to speak (and not more than that, it could also shorten notes whose duration extends past the start of the next note). I find myself doing this manually note by note pretty often. Kind of tedious.

Try changing your snap setting to "Last touched".
This allows you to work on notes of the same size and then go to the next notes of same size.

The Event Inspector in your control bar allows for such changes to selected events:-
Event start time
Event(S) duration(s) This can either be +/- an amount or a percentage or a fixed value.
An Events Pitch
An event(s) Velocity to a fixed value or +/- an amount.
The MIDI channel number.

See this current for all SONAR version
https://www.cakewalk.com/Documentation?product=SONAR%20X3&language=3&help=EditingMIDI.02.html
 

Have you tried holding CTRL while adjusting the duration of the selected notes?

HI:)
 
In the Event Inspector in the field Duration just type in:
 
120% p.e or the percentage you need. this work for all selected notes!
 
Bassman.
 

Not sure if you can lengthen and shorten notes at the same time or simultaneously with a single command tho.