How to get good fixture onto insulated backerboard walls once in-situ?

Hi,

PLEASE READ CAREFULLY I AM NOT ASKING HOW TO FIX THE BOARDS THEMSELVES TO THE BRICK/STUDWORK, I AM ASKING HOW TO FIX INTO THESE BOARDS ONCE THEY ARE IN SITU.

I'm planning on dry lining my bathroom with Marmox insulated tileable backerboards. Thinking of using boards of a thickness around 12-20mm, haven't yet decided. Please do weigh in on this aspect too! Where they are lining external (solid brick) walls they'll be fixed with some kind of foam gun adhesive or dot 'n dab, and where they line stud walls they'll simply be screwed. Some areas will be tiled (around bath and above sink) and some will be plastered and painted (in remaining areas).

On these walls I need to make get a good fixture for hanging items such as an LED mirror cabinet (quite heavy), tying in fitted furniture, framing for concealed cistern, shelves, loo roll holder etc.

How do I get a good fixing into these boards, possibly through tile in some cases? Do I use a regular plasterboard fixing (fischer etc)? Do I need some kind of special rawl plug? The boards are made from a kind of foam, quite different to plasterboard.

In the case where there is solid brick or stud behind, do I just drill all the way through the tile, board, air/adhesive, and fix into the brick or stud with a long enough screw? I'm worried the screw will be quite far away from something solid and act as a lever, putting stress onto the intermediate materials. Also, could this cause cold bridging?

In any case where I am making penetrations through tile/insulated board, should I attempt to seal the hole with silicon or something? Was thinking of pumping the hole full of silicone before driving the screw home through it.

Thanks again DIYUK.