I lucked upon the answer; for anyone in similar situation:
Although Administrator didn't logon in normal mode, it did in safe mode and accepted the password I'd added. Then I went to netplwiz and...

Dear all, this may or may not cause hilarity, but I've boxed myself into a corner:

1. Win10 on my laptop; got fed up of constantly having to enter password.
2. Googled non-password start and...