(In case anyone else has run into this problem, let me just describe how I fixed it)
I wanted to reset my macbook before giving it away (have done something similar before, for ipad/iphone etc over time)
This time, the "erase helper" got stuck asking me for the "Find My" password for an old icloud account (I had changed the email of my Apple ID a few years ago)
In almost comical "ye olde windows clumsiness" fashion, there was a modal window saying "enter password for Find My", with only a Continue button.
Searching, I found an Apple help page describing how I need to override an "Activation Lock"
There was a web form but it needed details of where and when I bought this (this was 4-5 years ago, don't have that receipt!!)
Note: This isn't a "real" apple ID (confirmed by trying to reset the password for it and being told it was invalid)
Fortunately, going into Recovery Mode, and triggering "Erase ..." from there allowed for a different path
It still stalled at the "password for the old icloud account, but it had another option => entering my previous local user password
This resolved the whole issue, and allowed me to go ahead and reset the machine
So, happy ending, but an example of some surprising corners that our secure systems lead us into.