I've not timestretched in SAM SE, but it looks to me like you want to use either the Object Editor, Stretch Length or Stretch Factor fields, and Standard mode. Alternatively, you could use Offline Effects | Resample / Time Stretching, Standard Algorithm, and specify the Stretch Factor or New Length. Just make sure you leave the Pitch fields at 0 and don't change the BPM fields. Sorry, I can't provide any more detailed "how to" than that...but it should be reasonably straightforward to implement the theory, I hope.
As for s/w, it's a very personal decision. Most of the commonly used multichannel audio editors do most of the same stuff. I've previously tried Wavelab and CEP/Audition, and finally settled on SAM SE. It's a steeper learning curve, I like it's interface and workflow much better, and it's tough to beat the bang for the buck.