Can I acquire a Windows 8.1 licence by free upgrade from 7 to 10, then downgrading
Solution 1:
Community wiki answer constructed from comments to question
My plan for the process is re-upgrade to 10, pull the license key, downgrade to 8 and use the license key, then upgrade to 8.1 via Windows Store, and end up on 8.1. [...] Is this possible?
No, your plan is not possible. Nowhere in that process do you acquire a valid license (key) for Windows 8 or 8.1.