Can Apple Motion export with alpha to bring into iMovie?
Yes, Motion can export with transparency into iMovie. I have used this exact procedure in the past and can confirm it works great. Using iMovie's picture in picture with some resizing retains the alpha and lets you overlay whatever you've exported from Motion into iMovie.

The linked question's answer is from 2011 and things have changed a lot since then, namely the introduction of the 'Advanced Tools' which include picture in picture (and have recently been abolished and integrated directly into the workflow). This means you don't have to 'splice' your video and work on it externally — it's all integrated pretty smoothly.