I'm a little unsure on what a judge normally says here. It'd probably be something like 'the court finds the defendant guilty' or some such. Rumia's just saying 'guilty', so replace with the shortest 'correct' line if necessary.

Also, is there a standard for Rumia's 'so- na no ka-' line?

Usually it's as you wrote with a tilde at the end. (Is that so~?) I don't know if that's the standard, though.

The darkness has spoken.

The defendant is hereby sentenced to death, to be carried out immediately as it is lunchtime.