Hi m8
Welcome
If you remove all waste from your supporting walls I would think you could replaster board the area making it structuraly sound. If its normal straw board the plaster board can be screwed to it to fix. The boards will not require skimming/plastering.
For a shower there are 2 ways of tanking it :
Firstly tanking the whole area, the best system would be a tanking membrane off the roll which fixes directly to your plasterboard with flexible tile adhesive. Putting this on the walls covering the whole of the wet area and the floor below your base. The membrane should over run both areas.
Second method using the same membrane off a roll, Install your shower base and fix a shower seal such as Sealux around the base then covering your walls with the membrane as above but finishing it on the shower seal. Tile over with any water resistant adhesive prefereably a powder mix.
Both of these methods will give a seal for any type of shower system, being based on the off the roll membrane it will be a very secure job.
Tiler
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