Hi there
Re point 1 - Yes DPM's that have a break in them are very difficult to repair, Do you have the room to lay a small dimpled profile dpm or a grooved say 2000 gauge flooring membrane over the existing cloak room floor? If so you could prime the soil pipe with a bitumen based primer...
Yes there is a membrane that would fix your problem very easily. The product is known as Newtonite which is a 3mm dimpled hdpe membrane which you would mechanically fix to the internal face of the single skin block. This creates a small cavity allowing any dampness to vent behind and a meshed...