“But if they fired a missile at a Ukrainian SU-25, missed, and the missile re-targeted itself onto the passenger plane, it is a somewhat less egregious mistake and something more easily done by an inexperienced launch crew.”
Two things.
First, air defense missiles don’t work that way.
Second, it is much more probable that the launch crew was Russian air defense, not some unskilled bunch of rubes.
It depends on the type of missile that was used and even on the upgrade status of the “seeker” component.
http://www.ausairpower.net/APA-Legacy-SAM-Upgrades.html#mozTocId532833