Can Maha Bharani Shradh replace all other Pitrupaksha Shradh days?

Not entirely, but Maha Bharani Shradh is considered so powerful that it can provide spiritual benefit equivalent to performing Shradh at Gaya. The Matsya Purana states that Bharani Nakshatra is directly ruled by Yama, the lord of death and the Pitrus. Because of this, Shradh performed on Maha Bharani carries amplified merit. However, if you know the specific tithi on which your ancestor died, performing Shradh on both that tithi day AND Maha Bharani is ideal. For those unsure of the death tithi, Maha Bharani serves as a powerful alternative to Sarva Pitru Amavasya.