Hi Edriano,
The modulator rate is determined by the SPEED bit setting. Fmod is equal to fosc divided by 128 (SPEED=0) or 256 (SPEED=1). Your crystal determines the fosc frequency, so if the crystal is 2.4576MHz and your SPEED=0, then the fmod is 19200Hz. If your decimation is 0x080 (128) then the data output rate is 150Hz. So your calculated outcome is correct, I'm not sure where you came up with 54576/128. I would check that you are actually writing 0x80 and not decimal 80. 19200/80 is about 240 sps.
As far as changing the mux inputs, the sooner you can do this the better for best settling response. You can reset the digital filter by issuing the DSYNC command. This has the same response as starting a new conversion cycle. So for consistent data response you can read the data result, change the mux channels, then issue a DSYNC command.
Best regards,
Bob B