Oh, I've read "Der Katte Prozess" some weeks ago; I didn't post about it since it offers nothing new. It's basically Fontane's final argument from the Katte section in Wanderungen about the lawfulness of FW's actions, with some added Fritz hostility and some early 1980s "kids these days" grumpiness. (The booklet was originally a speech given to a historical society.) (The grumpiness is about the fact honor and giving your word has no meaning anymore.)

If you want a summary:
- yes, FW was brutal, but he was well intentioned
- he was in a horrible psychological and emotional position with his own wife and older children conspiring against him with foreign powers and had to be afraid his entire kingdom would be destroyed if a successor capable of selling out before he even got to the throne came to power
- FW, not the tribunal, acted according to Prussian law at the time
- Whereas Fritz later overrode law if the whim took him (see also: Trenck, see also: Miller)
- FW was the best "inner king" Prussia ever had, and never started any wars
- FW had a conscience, Fritz had none.

No, Doris Ritter isn't mentioned. (The kidnapped long fellows aren't, either, though if they were of course that's a double edged argument, since both FW and Fritz were guilty of gang pressing soldiers.)

Is that a correct understanding?

Yes, though even those who voted for death offer arguments for clemency as well if the King were so inclined.