Building envelope with EWI and upgraded warm roof - drying out

[ex-70s-GP-surgery]

Hi all
I've been reading on here for 18months, but yet to get underway with my retro-fit upgrade. There has been a lot to get my head round. I love perceptual reasoning, but it gets stressful where I might be paying the price of getting it wrong.

My building is a 1970s built ex GP surgery. Most is a 100mm stud wall timber frame covered poorly by 20mm of polystyrene then 100mm split-faced dense concrete.  In this 70's build the 20mm of non foil-faced sheet polystyrene is where a cavity would usually be. There is no insulation in the 100mm timber stud. There wasn't much thought to insulation! The 70's era sections have no weep holes or any kind of ventilation. My existing roof is a flat warm flat roof with 60mm of insulation. The fascia/soffits are empty with no insulation link to the roof insulation. 

As I'm in Scotland thoughts have been to do 100-120mm of eps EWI system. Could I add a minimum amount of insulation to the existing empty timber stud and at same time use membrane on studs to limit moisture ingress? I don't want to trap any moisture in there that finds a way in through poor airtightness of the inner skin.

I would take off the existing timber fascia and soffits and link the top of the EWI up to the roof deck. (Good depth with layers of PIR back filled with foam to protect rafter ends) The roof will also eventually get 100-120mm of extra insulation build up. Ok so I'll have an envelope with much improved airtightness. In a retro fit I think that internally it would be nigh impossible to create an airtight seal without wrecking the rooms.  Will the thickness of the EWI mostly protect the inside from low temperatures and condensation?  I've spoken to one EWI installer who wanted to put trickle vents all over the outside just to be sure. Sounds like an expensive cosmetic render?
Rooms are generally 55%RH and I have a dehumidifier for washing.  I'm thinking (bathrooms) 2 X humidity regulated fans and if sensible some heat recovery ventilation as the airtightness increases. 

The second wall in my attachment is a different makeup. It is currently a partial fill cavity. I looked at filling the remaining half of the cavity and a woodfibre EWI system or just EPS EWI. There is currently 70mm of PIR IWI on a section, but that could be removed if it makes interstitial condensation more likely.