Hi Tao,
1, Yes, trigger mode FIFO can be used. You can use the corresponding interrupt to trigger it.
2. 55uA is a good assumption. The lowest current could be 30uA but this cannot be guaranteed for every single part.
If power consumption is very important for your application, you can check ADXL362, its power consumption is only around 2uA @ 100Hz output data rate.
BR
Neil