how to prepare 2 level surface for tiling, help please

[happyhero]

Hi I am helping my mother in law out as she cant afford much but wants to do her bathroom up. Its a 1930's ex council house. The council tiled it last, years ago and I have found that most of the tiles are coming off really easy and unbroken, in fact sometimes i knock 1 off and 3 fall off, almost seems amazing they stayed on there up till now. It makes me wonder what they have done for it all to come off so easy.

Anyway in some places they are fulling of what I think you would call the skim coat ie its about 2 or 3mm thick, so that i am ending up with 2 levels. Underneath appears to be a cement based scored coating, would that be a sort of render?

I have scraped the loose stuff off, but I worry will the remaining firmer stuff support tiles? It was tiled only half way and this is where the plaster has come off, up higher when I tap it, it seems ok to me. The render surface thankfully appears to be very level.

My questions are

1. Do I have to scrape all the skim coat off?
2. will the render surface be ok to tile on?
3. what coating should I apply to the render surface?
4. as it is being tiled do I need to level it or can 3mm be covered by applying more adhesive?
5. If I should level it what should I use?
6. Out of interest what do you think the council did when tiling this bathroom for the tiles to come off so easy?
7. anything else i need to know?

Any help appreciated.