With all the other hoops you are being asked to jump through it would be interesting to check when Windows Update has been checking for updates the next few days. Start, All Programs, Windows update will show you the last time windows checked by time and date. That doesn't mean it made the update at that time since it is on the BITS download system which only downloads when nothing else is going on, pauses if there is, and resumes the next time the pc is on. Defender, which should be off since you have McAfee, and a few other windows programs update the same way.
What home page do you use in your browser?
A few of the news channels you mentioned checking as well as the Hughes home page have way too many large changing pictures and little videos running on them these days for limited broadband.
Even the home page of Microsoft Bing has far too many images. If you leave any of these open in a tab while looking at others or nothing there is a lot of usage. A search engine page that is very plain and mostly text is the way to go. They are getting hard to find as Google Search is adding more junk all the time.
What email program do you use? Some of them check for new mail every 15 minutes but some almost instantly whenever they are open. Send yourself an email and see how long it takes to arrive back in your inbox with the program open.
McAffee should have a History Log and/or a Performance graph that will show you everytime something tries to come through it. You will be amazed at the activity that goes on in the background.
It should tell you each event and when it happened. If you have some of the common malware trying to gain access it may block it but there will be activity ( usage) every few seconds. Actor PID is a typical one. You can look it up on the web and see how much trouble it has been for many folks that are completely unaware it is trying to mess with their computer.
We all think nothing has changed in the way we browse and use our computers but the sites we browse have become more and more cluttered every year and the size and frequency of updates to windows and other software we have installed has become ridiculous, at least in my opinion.
Then just check your data usage before going to bed and unplug the LAN cable from the back of your modem. Plug it in when you get up and check the usage again.
There is a delay in the meter so if you just finished downloading something big prior to unplugging a significant difference might show but if you are just browsing there should not be much difference. Doing that every night for a week will give an obvious conclusion as to where the problem is happening without jumping through all the other hoops.
I thought you were reporting a big drain in the daytime while you were just browsing.
Have you set McAffee definitions and program updates to not happen automatically?
Sorry for my mistake.
When your data went from 80 to 46% two days ago, could you post screen captures of your McAfee history log during that time. It may take more than one screen shot to cover the period unless you can make a .pdf print of it. That should show what was coming in and going out during that time.
Double click the McAfee icon in the task bar or on desktop and select all history.
I only used McAfee when it is pre-installed on a new computer until the time runs out, but that is the way it worked for me. Check the above site to see how to do it and use the other features. You could also look in Help.
Go back in time to the incident the other day and also look at why you lost data allowance today while at work.
Your screen captures are a bit small for my old eyes but Charles McCool appears to see them ok. If you can make a .pdf of the list then the text size is easily enlarged.
Photo captures in .png or .jpg do not enlarge well.