The knowingly specification is going to be a tough one to prove
Difficulty in proof is up to the Texas AG to decide and worry about. Broden has a point, Petersen sits as a judge, setting bail. Does he know how to follow Texas statute in setting bail, or not? Petersen's announcement that bail was set to "send a message" is, well, not cool for a judge, and outside of the bail statute.
The "knowing absence of probable cause" is a little more tricky, but the magistrate is supposed to know probable cause, and ought to know that an affidavit that does not particularly state allegations against the accused is, as a matter of black letter law, deficient. Not quite as neat and tidy as the Texas code for setting bail, but probable cause is something a magistrate judge is required to know, else they are incompetent to perform the function of approving warrants.
Paxton can blow this off, "got your letter, thanks." Then six months later say "investigation complete, no prosecution." He has no obligation to explain, any more than any prosecutor has an obligation to explain. It's all political.