detect INACTIVITY and go to sleep //that works if I set line 90 to 0, correct?
No, that change would deactivate the detection of inactivity completely.
But as far as I understood the datasheet so far you should leave line 90 as it is but once you detected inactivity you should deactivate that interrupt source and then go to sleep.
DOUBLE TAB is detected -> wake up //that works if I set line 93 to 1, correct?
Yes and no, if you leave the rest of the program as it is, it will wake up by every event (TAB also).
If you want to get above functionality you have to change the interrupt configuration of the sensor chip for every event received. This is especially important if you put the Arduino to sleep as any interrupt will wake it up.