ᐅ Sealing the base area for earth formwork and gutter installation

Good day,
I have a non-basement building constructed in 1985.
The foundation slab is made of watertight concrete, and the exterior walls are built with Poroton blocks.

Unfortunately, in some areas the Poroton blocks extend beyond the foundation slab, which causes moisture to rise despite a bitumen membrane between the slab and the first row of blocks.

I originally planned to seal this with a bitumen coating, but the foundation slab is uneven in one spot because it was formed directly against the ground instead of using smooth formwork.

At another spot, the foundation slab seems to be too thin (?), so cut Poroton blocks were used to build up the wall below ground level.

Do I need to prepare and smooth this area somehow, for example with a cement mix? Or can I simply apply a thick layer of bitumen coating and consider it done?

Additionally, I am wondering how to seal the area where a rainwater downpipe, embedded under the plaster, emerges at the base of the wall.

I hope I have clearly described the issues.

Thank you very much for your help.

Thorsten