I hear you loud and clear. You mentioned "few months of suffering another fatal crash" so I assumed that meant you were crashing in Fusion 360. That's why I mentioned the error reports.
Unfortunately, I do not know why the MAS (Mac Apple Store) is not seeing the update but the Fusion 360 client wants to update.
You're seeing this error message when you launch Fusion 360, right?

If you select work offline, can you then select the Diagnostic Log Files command from the Help menu?

If so, could you upload a copy of the ZIP file here for review? The log files would allow me to confirm which version of Fusion is installed and potentially tell me if there is another issue at play.
The easiest and fastest solution will be to go into the Applications folder in Finder and manually delete Fusion 360.app (Right click on Fusion 360.app and select Move to Trash). For good measure, you can run the Fusion clean uninstall tool I mentioned above to remove all other Fusion 360 related files from the computer.
Last, re-download Fusion 360 from the Mac App Store. Or install it from the Autodesk website, by going to the below website and selecting Download Free Trial.
https://www.autodesk.com/products/fusion-360/overview
Your account and data is all associated with your sign-in information. It will not matter where you install from. You will gain access once the local client is up to date.
I hope that helps. Please let me know if you have any quesitons.
Thanks,
Nathan Chandler
Principal Specialist