+STEM on Google+ Community share your story event. It's messy and doesn't work as you might expect it to on first glance. The short version of the story is this:
1. You can upload videos into events, but it seems they have to be encapsulated in albums that already exist.
2. You won't see the video in your album, but go ahead and add the album to the event anyway. The video will be added.
3. Each photo in 'Photos from Posts' is its own album. Consequently, the easiest way to share a single video to an event is to first share it to your stream to get it into the 'Photos from Posts' album and then, add the video to your event from there.
Gory Testing Detail
Now, here are all the gory testing details so you know what we saw and also so you'll know you're not going crazy when you run across these problems.
OK, here's the latest. I tried to download a video from my G+ account and then upload it to Google Drive. It came down as a .mov file. I copied the file to a Google Drive folder and waited for it to sync. Then, I chose the video from my Google Drive folder using the add photo dialog from the event. The video icon only displaying as a grey box, but I was allowed to select it and add it to the event. The video didn't play however. The same video in my Google Drive folder also didn't play.
I deleted the video from the event and started over. This time, I clicked on the Albums tab of the dialog. Then I selected pictures from posts and selected the same video which I had posted earlier today. The dialog says 'no photos', however, the 'add album' button was enabled. I clicked it and the video was successfully added to the event with sound.
Now, here's another tricky bit. I copied another video from a private to a public album. When I select this album, the video doesn't appear. The video does appear in the event in the normal interface however. And... if I just go ahead and add the whole album to the event, then each picture is added individually, including the video.
To test things out fully, I shared the video from my publicly shared album to my stream. The next time I tried to add a photo to the event, I selected 'Albums', then the album 'Photos from Posts', and there was the movie I'd just shared to my stream. I clicked on it and was taken to a seemingly empty album where I was able to click the 'Add Album' button and the video was added to the event, (see the first case above).
I'm now trying to upload a video directly to the event. After a very long upload time, I got the 'server rejected' message.
One last thing. One of the videos I uploaded to drive tonight is now working... sort of. It was originally a 32 second video and it is now a 30 second video. The other video I uploaded to drive tonight has yet to work.