As far as I know, there is no such policy to mandatory filled in security training list in SharePoint online.
More information about SharePoint Online Security for your reference:
For your issue, I will assume Survey in SharePoint 2013 will be good options for you.
Quote from your requirements:
someone from one organisation mustn't be able to see data from
another organisation 2) they shouldn't be able to see data from their
own organisation if they leave their organisation or change their job
It is a problem more related to ...