How does Obi-Wan know Darth Maul's name?

I really do not think the Jedi would have tortured anyone.

Most likely the new supreme chancellor had his own people conduct the investigation and interrogation. When the report was sent to the Jedi all relevant information to the sith was omitted. Darth Maul's sith name was no longer sensitive information so he most likely let that pass to give the Jedi something of note.