Well, it seems to me your IE11 is screwed up.
IE11 is a really good indicator of general windows health. If IE11 fails to work and all other browsers work (Chrome, Firefox, Opera, Safari, etc.), that computer still needs servicing.
I had no problem going to the Microsoft Catalog page on IE11 on multiple W10 machines I have using IE11, Edge, and Chrome. My next question would be, have you tried downloading the update files via IE11 on a
DIFFERENT machine running W7/8.X??
If not, you might try that and then apply the file via DVD or USB to the computer you are trying to fix. If that works, then you most likely have a software problem with the W10 kernel on that original computer (as IE11 lives in the Win OS kernel now), or you have a failed hard drive for your C: bootdrive; no matter what drive diagnostics you used to test them with are.
I'm leaning towards Rich's suggestion of a bad Mobo; but it could be as simple as using install media created on a another computer using a known good copy of IE11 as I said, or even a hard drive. If you haven't yet replaced that drive, you should do so, and repeat the Clean Install as we suggested. Drives are way cheaper than a new Mobo as you are well aware; that's a thing of last resort to try on this problem. But, if you can't get the post AU-updates to install, you definitely still have something hinky there in that computer.
All these problems Rich is having with the MS Catalog download page kemical provided to us are quite astonishing. I already asked him if he wanted me to come over to his place and fix it, but I don't know where he lives and he wouldn't pickup my plane tickets and hotel!! LOL!
Best of luck,
<<<BBJ>>>