The effect can be done with iMovie 09. At least the first sequence you mention. It's a little tricky.
First of course you need your character shot against a green screen. Lighting is critical for that and you can find tutorials on line about how to do that.
Second you need your still scene. You should prepare it for the aspect ratio of your video. You then need to make a copy of that image to use as the foreground. Edit the foreground image to remove those items that you want to be seen as the background. That area has to have 100% transparency. You'll need to find an image editing app that can do that. Save it as a PNG. I tried a TIFF and JPEG and they didn't work.
Now that you have all the pieces, create a project, first add the background image, then add the green screen character. If the green screen affect isn't perfect, you might be able to use the "Video Adjustments" to fix it. Now export that project as a QuickTime movie file. Chose an uncompressed codec for what I would hope to be best results. Import the resulting file back into your event you are using for this scene.
Create another project and add the new video to that, then drag the foreground image over that video choosing "Cutaway" from the pop up options. Some timing adjustments might be required to make the lengths of both sources the same. Do another high quality export for use in your final video. That last part is required if you are going to do any further processing on the scene such as a transition.
