Porous copings & inadequate pointing- new property

I have a new build property. Wish I didn't but I have now. Built 2 years. It's become evident that last two winters the pointing/mortar in the garden walls is inadequate (brushes out in summer and is saturated with water in winter and is now crumbling away). I can remove it in places with my fingers. Looks a real mess and wall is saturated with water. Bekstone stone house and walls.

Garden walls have been capped with once-weathered coping stones - the stones themselves are porous!! Unbelievable! They soak up the water and it travels down through the sandy mortar making mortar and stone very wet. Joints between copings is poor mortar too and really joints are too wide.

3 questions:

1. If I have walls repointed to replace the poor sandy mortar where needed, remove copings and put membrane beneath copings & then seal copings with stone sealant will this solve all my problems?
2. An upper part of a wall was re-pointed recently beneath copings (see 1st pic) no membrane and remember copings are porous)). Since it's been re-pointed but the coping issue has not been addressed the wall is even MORE wet and it's massively noticeable. How would re-pointing have affected this? How can I dry the wall out?
3. The same wall has been repointed with a very dark mortar recently and I noticed, before the upper stones were saturated that these stones had become a different colour to the yellow colour of the stones beneath. They are now a grey-yellow. So, I have two bands now of stone colour!! Why has this happened?

Developer does not want to know but I'm in my 2 year period still but they tell me the walls meet minimum standards just wear and tear. Obviously rubbish.