there might be two issues - one that you can't start (none of the logs here are
And another one of this failing mysql upgrade.
Maybe the latter blocks the system from resolving the former by updates.
And I can at least help for the latter.
>From your logs:
That will make the mysql upgrades fail.
This seems to be manual removes of the file, but not purging/removing the mysql
With that on the next update it tries to restart without config and fails.
You need to re-setup working conffiles  or fully remove mysql.
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package mysql-common 5.7.21-0ubuntu0.16.04.1 failed to
install/upgrade: subprocess installed post-removal script returned
error exit status 2
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