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How to (backer) board this tricky wall part concrete/part stud

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A few basics to start. I am planning to use 12mm Hardie backer board to support heavy 12mm 600v1200mm marble tiles. For most of the area the backer board will be screwed straight into the wall studwork. Actually I do have an initial question on that......from what I can see from Hardie's own install instructions you don't need to use any washers like this for example - https://www.protilertools.co.uk/categories/tile-backer-boards/backer-board-washers

They indicate you just use their screws which I have. Is this right as other manufacturer's of board appear to promote the use of these washers!?


Anyway back to the main issue - the back wall of our shower enclosure is part concrete with stud above - see the photo - the concrete retains our upper floor slab. Its obviously not completely straight so the first issue is how to attach the board to it perfectly level in all planes. The good news is that its not out that much anywhere - maybe a max of 10mm across the span. Hardie make their own product Hardiefoam which I think is used for this sort of purpose - https://www.protilertools.co.uk/userfiles/files/ef495d2518836cc0409a919912f75d60_HardieFoam-Adhesive-Data-Sheet.pdf - so my next question is would be suitable for adhering the board to the concrete and where any board overlaps onto the stud then presumably I need to pack out the stud so its level with the board to support it and then screw that section? 


The final thing is that the foam product only supports 50kg/m2 so Hardie say to use 9 masonry wall fittings per board to support up to 200kg. I suspect I am better to do this for these heavy tiles so the final question is what fittings to use and what to fix them? Do I wait for the foam adhesive to fully cure and then drill holes through and into the concrete with the SDS drill I have? 


Then the final question is what sort of fixings to use? I assume stainless steel but would something like this do - https://www.screwfix.com/p/friulsider-a2-stainless-steel-hammer-fixings-6mm-x-60mm-100-pack/720jy - or any better suggestions?




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Haven’t read all 

Screwing the board with drywall screws is fine As most of the weight is supported by the floor Solid wall Bed the boards on with tile adhesive Then once set drill every 300 mil 50 mil in from the edges and use BG knock ins 


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