1. I have created a small code example for checking these two conditions. In the example I am using LED's to determine whether the I2C_1_I2C_SSTAT_WR_BUSY (Blue led) and I2C_1_I2C_SSTAT_WR_CMPLT (Red led) conditions are are triggered. I am setting a flag in the I2C_1_I2C_ISR_ExitCallback() function. Depending whether the flag is set or not the I2C_1_I2C_SSTAT_WR_CMPLT or the I2C_1_I2C_SSTAT_WR_BUSY status is checked in the main loop.
I have used the Bridge control panel to send data, and I can see both the LED's glow, i.e. the Red led followed by Blue led.
Please have a look at the project, in case you still face an issue, please attach your project so that we can have a look at it and try to recreate the issue at our side.
2. The I2C_1_I2C_SlaveCompleteCallback() will be called when the master completes read/write operation on the slave. You can use it, if you want to do any processing when this event occurs.
3. The I2C_1_I2C_SetSlaveInterruptMode() can be used to configures which bits from Slave interrupt request register will trigger an interrupt event.
The I2C_1_I2C_ISR_ExitCallback() can be used in your case to check 2 conditions and set certain flags.