Good news (sort of....)
I now know that the problem has nothing whatsoever with my phone or my MS account.
Today I had access to my son's laptop, and was able to verify what I was beginning to think - the problem lies somewhere in my own computer. On the laptop, we were able to sign into my MS account and start to download an App. I am confident that I will now track the problem to its source.
So again, thanks to you all for your expert advice - I'm just sorry that I started us all on the wrong track in the first place.