"František is not interested in that."
Translation:O to se František nezajímá.
It is a really weird word order. It would only be acceptable if the sentence continued. Something like "frantisek is not interested in this BUT he is interested in ..." in Czech that would be "František se nezajímá o to, ale zajímá se o..."
Again, unnatural order of words. It would mean that it is not Frantisek who is interested in it but somebody else. But we are missing the information on that other person.
O to se nezajímá František, ale Kateřina.
This is a hard translation (SVO is considered wrong here). Am I correct in assuming that the solution is intended to stress the in that part? As in "František is not interested in that!"?
I do not think there is any special stress implied here. The stress, though not too strong, is most likely on "nezajímá" here, so "is not interested."
You just have to follow the second position rule for "o to" when needed (i.e. not always, it can be also in the frst position). That is the reason why the SVO does not fit well here, because this specific object has special rules.