>> End result: major borkage and a broken boot
Yeah. For mint, the recommended and fully supported upgrade path is a fresh install.
If you upgrade it the other way, it's risky, and you knew it, you checked the box with the red warnings before.
Anyway,the fresh install on Mint is two times quicker than the update, so why bother with the update ??????
My recommendation (through GUI):
On first install, create 4 partitions :
sda1 : root, 40 GB
sda2 : empty, 40GB
sda3 : swap, 4GB
When upgrading, install the new system from fresh on sda2, then you can boot both old and new OSs, and keep all your data in the right place
Next time upgrade sda1
With that you always have two working systems and never have to break anything immediately.
Update ? 20 minutes of work(including installing a prepared list of packets), 20-40min of unattended working time
Also, use and abuse backups.
I never had a single problem with that strategy on ca. 15 systems since 4 Years. It upgrades faster than any other OS, and you can revert....