What happens (long term) if you trap breathable membrane against a wet stone wall with a layer of IWI?

The situation is that the wall drops below the door threshold level and is below ground on the outside (street pavement) but is open to the space under the suspended ground floor.

Because of this, over the decades ( the original pavement would have been below the poured dpc one assumes) the original joist timbers have rotted. To remedy this we built a dwarf wall inboard of the wall to support new timbers with sufficient space to drop the IWI all the way down passed the joist ends and into the underfloor space.

Since this has taken place over the course of a few years (ahem! http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11175&page=1#Item_9) ideas have changed as we've moved on. In the main room (now, I can see this clearly but I hope that I can describe it so that it can be pictured) the IWI goes down the wall below the joist level, finishing below the joists but a strip was left out at skirting board level. The membrane was drawn through under the joists and above the supporting dwarf wall, pulled up above the joist ends and secured to the wall with the final strip of IWI at skirting level. However, at the door in the hallway, there's no wall at skirting level!

So, I was thinking of drawing the membrane through under the joists, up against the wet wall under the door step and then trapping it directly to the stone threshold with an improvised upvc capping. Insulation could then be slipped down into the gap between the joist ends and the (now membrane covered) wall.

All this is complicated by the presence of the dwarf wall and the question of how I properly seal the membrane to the rough stone walls in the corner where the joist is hard up against the wall..... another discussion!

But does anyone have any comments on the opening question?

Apologies as I don't have an autocad or similar so this is a basic sketch of the situation.
The membrane is Membravent 120.
The original dpc is a black bitumen type material and it is in the sandstone wall at a level that would have been just below the original joists and above the original outside ground level. It's now, unfortunately, 200mm below the pavement.
I was thinking of capping the step with a upvc board in the same way that external window sills are treated for EWI.

If it is actually against the wet wall I suspect it won't do well at all. As far as I know breathable membrane is not designed to be used as a DPM.

''Where is the DPC? What is the thick black line across the step and down the inside of the wall?''

As far as I can tell from the OP, the dpc is approx 2nd mortar jt down on the ext wall, roughly in line with the thick black line, which is the (proposed final route of the) breather membrane.

I'm sorry folks but I've reached the limits of my sketching abilities! I can't even work out how to get colour on this package to make things a little clearer.
The original joists were the same height as the new ones as they had to be level with the ones at the back half of the house. The original DPC level in the external house wall is just underneath the (sockets for the) original joists.
The thick black line across step and down the wall was my attempt at constructing a sketch. The remaining thick black line under the door represents the stone step that's on the wet sandstone wall.
You are right that, at present, I haven't trapped the breather membrane directly to a wet wall - in the main room it's isolated from the wall by a layer of vertical insulation up to skirting board level and is then adhered to the wall and trapped by a covering layer of insulation so that there is a continuous under-floor to attic insulation layer.
It was cold outside overnight. I opened up my experiment with the membrane on the internal doorstep and the upvc cap over it. I have discovered two things - 1. moisture can be felt on the room-side of the membrane but not as much as I had expected. 2. If you stop the warm, moist internal air from getting to the cold stone step it is drier!! So I plan to build up a sandwich of dpm, that thick "bubblewrap" underlay that you can put under flooring, the breather membrane and cap it with the upvc. It's the best I can do if I want the door to open. The gap between the joist ends and the membrane sandwich against the wall will be filled with more PIR.
The breather membrane incidentally continues under the whole floor and will be sealed at the edges to the rough walls with an as yet untested parge coat on the stone and a strip of upvc to trap the membrane to it. It's going to be a faff but necessary to impede the flow of cold air from under the house.
The IWI is cork, attached to the walls with Adhere Vit, fibreglass mesh to the internal faces and finished in lime plaster.