UnitClassification works and the target is cleared but after UNIT_DIED fires.
So if you had the mob targetted as it died the screenshot is taken as expected.
I can't think of a way to combine the 2 so that they complement each other.
The list will catch all rare kills included in it.
Trying to qualify it by checking that it was your target as it died (weeding out false positives: other peoples kills in the vicinity)
will effectively be the same as if you didn't use a list at all and only used the classification+target method.
What might make some sense is check classification after checking the list ('or' logic) to catch a rare that isn't included in the list (was recently put in game, or hasn't been encountered/datamined by wowhead or the list of ids included in the addon is incomplete).
I imagine this to be an extremely rare occurrence.
Flipping a coin would be an idea
Edit: Maybe go with the more 'catch all' (safer) approach of the list.
A false positive here and there shouldn't be an issue for the majority of users.