Just wanted to chime in a thought. I found this thread when I was experiencing a similar issue. I just activate project fi service (my first time on a metered data service) and the day after activating hit my 2gb budget. It was all caused by 'google photos' app. After doing some tests I'm pretty sure it's caused, at least in my case, by using Photos as the daydream app. It seems to be related to this issue logged against google+ a while back:
https://code.google.com/p/android/is...etail?id=52815
I might be wrong but it looks similar to some of your issues.