If you are having speed issues, please head to http://testmy.net/ and set up an account. Please connect a single device directly to the modem. No router. Then run some speed tests using the 12MB manual test size for download tests.
Please try to run a batch of three to five tests, each test spaced at least five minutes from the others, and try to run a batch in the morning, afternoon and evening. The point is to run some tests at different times of the day to get an average.
After you've run a few tests, please click on the Results tab and post your Results page URL. It will look something like the following, with your account name where it says "yournamehere".
You're welcome. Were you able to determine what was/is using the excess data, at least for the most part, or are you still in the process of monitoring your devices to figure it out? I know it's only been a few days and you might be needing more time to glean enough information, but I just figured I'd ask.
To add, if you end up testing your upload speed for whatever reason, make sure to use the 2MB size for that, not the 12MB size used for download tests.
Hopefully whatever it was/is will eventually be revealed. With that said, sometimes devices do odd things that we can never figure out. I'll give you an example...
I update my folks' computer on a regular basis because they have dialup and the updates on their Windows 8.1 computer would take them forever to do. Previously they had Windows 7 on the computer. I had a legacy plan at the time, which gave me a daily amount rather than monthly, and it was 425MB. They also gave one free token per month, up until you had a total of six. If you then used one, the next month they would give you another one. The tokens were worth a day's data.
When I would update their computer with Windows 7, I would do the normal updates, and update other programs as well. Then, with the computer just sitting there, it would start using data. I couldn't figure out what was going on. One day I ended up using four tokens. The total usage for that time ended up being over 2GB, and it took a long time as my speed was about 1.5MB. I normally would update it at night as the legacy plans had the "download zone," which was an unlimited period between 2-7AM that didn't count towards your daily allowance, but this time I couldn't do it during the night. This happened a few times, though never using as much as the 2+GB it did the one time. It would often use nearly 1GB, though.
Finally, after nearly pulling my hair out over the course of about a year and a half of dealing with this, I figured out that the downloads were being done through a program called BITS, which stands for Background Intelligent Transfer Service. It's a Windows program. It's designed to update things that don't update during the normal procedure. What those things are, I couldn't tell you, and I never did figure out what it was that was using all the data. I even installed a fresh copy of Windows 7 on their computer, but it would still do it. If they had high speed internet, they would have never noticed it as it would have done it bit by bit, day by day, but they gave it to me to update on a monthly, and sometimes bi monthly, basis, so it would all happen in one pop.
Finally I upgraded them to Windows 8.1, and although it still does it a little bit, it's nothing like it was.
The point it is, sometimes these devices do things we don't expect, don't know that they are going to do, and sometimes can never figure out. It may or may not be your devices, but in all honesty, I still tend to think it is. Let me say, though, that in no way do I think you're embellishing what's going on, nor lying about it. I 100% believe you. I just would not at all be surprised if one day, some time down the road, you come and here and say, "Hey. Guess what. I found out what was eating all of that data. It was such and such." It happens a lot, actually. We find it by searching and even by accident. Or we just catch it in action, so to speak.
Writing down what you are using may be a good idea for a while, just to make sure whatever happened doesn't spring back up. How are you able to monitor your usage on your separate devices? Do they all have monitoring programs?
I personally won't even use the cloud. Too much data needed and I can't control it. It just does what it does. I have mine disabled on all of my computers. The only backups I do is saving important data and programs and on a flash drive and doing monthly system images of my computers.
As for the phone reps...they can sometimes be...well...less than cordial. It's why I'm grateful that we have this place. Most of the phone reps are nice, but once in a while you get one that just irks that heck out of you. I've had it happen, too, though it was many years ago. I've not had to call the phone reps very much in the last seven or eight years, but did so more often before then.
Did you end up creating that private thread? My experience with the reps on here have been great. When something like this happens it's often difficult to learn what actually happened and the only thing that can be done is trying to be vigilant in adjustments to programs and such and monitoring it to see if it does or doesn't happen again. Basically not being able to correct what happened, but hopefully making it so it doesn't happen again. Hence all the monitoring advice and such. It's good that they gave you some tokens to get you to your reset date.
I hope it doesn't happen again, but if it does, with your vigilance you may be able to tell what's going on and stop it.
I wish you luck.