1. Got use membrane here but different with western ones coz theirs are for drywall.
Its not very common for bathroom more for roof. It will be expensive for one.

2. This one really depend the sifu tiler skill.

Membranes are typically NOT used in M'sia bathrooms/wet rooms. Typically a waterproof cementitious layer is applied as a waterproof barrier. Please see the following links for an idea of what is done here

Sika bathroom products:
https://mys.sika.com/en/home-improvement/ba...m.html#wet-room
Item 2 is the waterproof layer, and item 3 is the tape that goes around the edges and corners to make the wet room completely waterproof

Mapei bathroom products:
https://www.mapei.com/my/en/products-and-so...mp-environments
Item 4 is the waterproof layer, and item 9 is the tape that goes around the edges and corners to make the wet room completely waterproof

My experience with the work method in M'sia is the statement "orang belakang akan repair". Meaning something like the electrician will hack the wall, but he expects the plasterer or painter after him to smoothen the wall. Or the general worker will flatten the floor, but he expects the next worker after him to screed the floor with the slope, or the tiler to do it.

This is wrong. Everyone lepas tangan to the next person.

As an example, in US, the framer must make sure the frame is flat (generally ¹/₈" deviation over 1 yard maximum), or the sheetrock guy will refuse to do his job. The waterproof guy must do their waterproofing, and then do a ponding test for 72 hours, and it must drain completely, before the wet room is screeded flat.

Every step is important. If you are doing a major renovation, I suggest you hire a Clerk-of-works (locally "Kerani") to monitor the renovation is done properly.