Tiling walls in a shower cubicle

It’s alright to do so, in many countries they do that. Only think is to use green plasterboard and to be sealed properly. Including good grout .

You can. Ideally the plasterboard would be the green moisture resistant stuff rather than the standard plasterboard but saying that, if the work is done properly then the tiles, grout, and silicone would stop any moisture penetrating into the substrate anyhow. Saying this, if you have a bit of budget and the appetite to do things a bit more robustly then I would recommend ‘tanking’ the plasterboard with a kit such as the Mapei prior to tiling as this would provide a belt and braces barrier should the tiling fail. Hope this helps.

You can (but i wouldn’t) I would strongly advise to use aqua board or flexible waterproof membrane beforehand. Obviously this will cost more to do but can save you money and disruption in the long run

You can tile straight onto plasterboard in fact you can lay tiles 12.5mm thick and no more than 32kg per sqm and if it’s plastered it’s less weight per square meter of 20kg per sqm

Best practise is to tank the wall. This will ensure that no water can get to the plasterboard. Silicon is NEVER enough.