Maybe I take a different path, at the end of the installation, when the Q&As start, I skip the request for signing in with a MS account and on the next page, sign in with a local (David). At a later stage I resign in with the MS account. This name has not changed for me, in the File Explorer - users. Subsequently, none of the other problems you have experienced, have occurred. Why, I know not, but it does seem that the only fault is why you would have the two different usernames.
I cannot comment on the Store problem. I can only say that I have only two "Apps" running, both of which I was using in Windows 8.1.
But I confess that I am not certain what you mean by "Has any other past Windows 8/8.1 users like myself found that 10...