5.6 crashing immediately after a forklift upgrade

Title says it.

I am attempting to get us bumped from 5.5.50 to 5.6.31.
[*]Standard MySQL, not Percona (not opposed, haven’t tried, doubt the result will change)
[/LIST] After installing the packages, this is what gets dropped into the mysqld.log:


I’ve fixed the missing table errors, except for the temp table one. That does not appear to be the cause.

Things I know:
[]The data is a mess, lots of FKs, fields with data longer than the field should allow, etc
]At some point in the far past, mysql crashed in the middle of an alter (hence the missing temp table)
[]… which was fixed by the former admin by simply deleting the .frm+.ibd temporary files.
]I can downgrade to 5.5.50 and be just fine again.
[/LIST] Primarily, I’m just trying to get us into 5.6 so we can then head into 5.7 territory while an application rewrite happens (which is years off from completion).

Could really use some advice on what I should be looking for next. I’m at the end of my rope and standing knowledge base.