Could a character use Focused Spell with an Illusion (Figment) spell, for example minor image?
Would the caster be able to choose one of the enemies when casting such a spell to have the heightened save DC?
Focused spell says that the spell "affects or targets more than one creature". It seems that "affects" isn't actually defined. Would the fact that there are multiple creatures who might be required to make a save class as them being "affected"?
A silent image doesn't affect anything. Nor can you choose who it is targeted to (your allies must save to disbelieve as well), so it isn't a targeted spell so it disqualifies for the targeted part as well.
You can react to it but the image itself doesn't affect anything. Just because multiple people are viewing it doesn't mean it's a viable spell candidate for Focused Spell
Also I think the RAI for focused spell is for area effect spells.
You must interact with or study closely the illusion in order to even initiate or attempt a save against the image.
I would say NO it does not.