I have not got my 9250 module yet.
I use the 6050 with an external magetometer on a separate PCB, connected to the secondary I2C bus. It was a bit of a nightmare to get it to work. The arrangement looks very similar to the diagram on page 21 of the 9250 datasheet, except that the magnet device is now in the chip package. Also I was using the HMC5883L not the ak-whatever japanese one.
IIRC, there are at least two ways that this can potentially work.
One way, is to use the main I2C bus to look right through the 9250 and communicate with the magnetometer directly. I could never get this to work. I guess this scheme is the "serial interface bypass".
The other way, is to tell the 9250 to continually read the 6 data registers of the magnetic device itself, and copy the data into 6 of the spare registers on the 9250, and then you just read the data from those 6 spare registers on the 9250. It was a problem to get this to work, but once I got it working, it worked just fine. I guess this scheme is not the "serial interface bypass".