Does inverted sawtooth have similar harmonics to square waves? And why have some users said that with inverted sawtooth the Duty Cycle must be set at 49%?

Inverted sawtooth waves use every harmonic (refer to "Harmonics” in the Glossary) but square waves only use odd harmonics. The duty cycle setting is only necessary for the older UDB generator and is set automatically for it by Spooky2.