Why is “Send Movie to Final Cut Pro” completely missing from iMovie 10.1.3

Don't forget the built-in Automator actions for encoding. The video plugin is very nice and has pre-sets starting with 480p (VGA size) and up.

The Encode for: Greater compatibility toggle keeps the audio and video at a lower bit rate. Depending on what your tolerance for "less degradation" is, you might experiment with Higher quality if you see too many H.264 artifacts on the default setting. It's unlikely as the encoder has a lot of room with 1.5 Mbps as the ceiling for the video data rate.

You could set up a few services to test the various settings and then make a folder action to apply your favorite encode setting to anything you drop in that folder.

Automation is fun—you might even be able to set up a folder action to do your uploading as part of this process.

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.