I am a glazier and fit lots of showers and mirrors, you should not use silicon or sika for mirrors as it eats the silver backing over time, I use " mirror max" for this job. On drilling holes into tile, I never use silicon in the holes but I am drilling a 3mm that gets filled with screws or slight bigger with plugs, on any hole bigger yes then use sealant, most times you can't pick up the stud after the fact but the blue board is enough generally
Cheers