You summed it up in your Subject line. You can't use Warp Stabilizer and change the duration of the same clip. You could apply Warp Stabilizer, render it out, and bring the rendered clip back into the Projects panel. That's as far as my meagre knowledge goes, though.
I suppose the best way (that I didn't do) would have been to warp stabilize first - before doing the rate change and applying transitions and PIP and RGB curves...
I have used various stabilisers over the past ten years, and have always found that the best stabilisation results are achieved with the raw footage, unless this has very low contrast. In that case it pays to apply a levels effect first.
I always export my stabilised clips to an intermediate file using a "lossless" codec before importing that new file into my project to replace the original footage. I use the free download UT codec for this. I have tried Lagarith and the free Avid DvxHD, but have had problems of one sort or another with each.
Speed changes never work well when the clip needs stabilising, in my experience.
You're correct. You can's apply a speed change to a clip with Warp Stabilizer applied. Here's an easy workwaround.
You can also apply Warp Stabilizer then nest the clip and apply the speed change.
Thanks for the help - looks like nesting will work great for me!