Problem re Windows 8.1 activation

I am trying to update an older but decent Toshiba laptop for daughter to get started on a Diploma course required by her new job.  I have been on this problem for 3 days now and spent about 12 or so hours trying numerous solutions.  The situation is now more urgent.

In summary, Toshiba Portege Z930 i5 10 Gb ram, 120Gb SSD circa 2013.  I acquired the PC as part of a redundancy package running W8.1 at the time which I subsequently upgraded to 8.1 Enterprise then to W10 over 2016 using a MSDN subscription which I purchased. All running well for about 2 yrs including Windows Updates until I retired it.

I started to apply Windows updates - 2 rounds but I could get no further than 1803. Then suddenly Windows crashed deleting boot sectors and C: contents!!

I tried installing 21H1 from bootable USB but could not get past the first reboot when the install process just restarted without reason - 3 attempts.  The same thing happened with the 8.1 Ent from the archives.  I would really like to understand what causes this behaviour.

I then attempted to install from 8.1 "full" and 8.0 Pro  CD's from a second computer which was scrapped prior.  Another problem - now I could not install due to a "missing" media driver. 

Some research led me to WinSetupFromUSB 1.9 which enabled me to create bootable USB with archived W8.1 Ent ISO which enabled me to install it.  I have no idea why this product would work (with something like but not PLOP) as opposed to the ones created with Rufus.  Again, I would like to understand the difference. 

However, I cannot activate it but I need an activated licence to subsequently install W10.  Thought I made a breakthrough last night by using slmgr.vbs /ipk <key sourced from ProduKey.exe> because it removed the Activation Warning.  This morning, I tried installing 21H1 again but it failed saying my PC did not have an activated key.  Looking at the desktop shows the Activation Warning again.

So now what??