I've just had someone out who advised everything exactly as you have in this thread. So much so I wonder if you are him.
Part of me wonders if the house needs to expel water through the outer skin? If this doesn't happen then fine, obviously pliolite is the best way forward. But pliolite I've heard isn't "breathable". Could it be possible that the wall is damp and has "thrown off" the paint coating on there at present, resulting in the situation I'm in?
That's my only worry and just wanted to bounce this idea off you as you seem like you know your stuff
LOL! No it wasn't me.
It could well be that it was painted when it was wet with the wrong paint and as you say it has thrown the paint off or the paint has been compromised in the past and water has got behind it. I mean yes it is important that the outer coating is solid hence the suggestion to use Pliolite. Shellac based interior paints like Thompson's One Coat are breathable so they will let the moisture slowly evaporate in to the house. But you are right that no masonry paints are breathable. I have never found this to be an issue, and I have treated some very damp walls. Your problem is a bit unusual though because you have injected foam insulation which slows down the drying out process. Normally I would suggest a few small vents in the outer wall but that's pointless with foam. I think the main thing here is we aren't entirely sure where the damp is coming from and to treat all the potential sources as fast as possible to stop more water getting in and the then the walls should dry out very slowly by themselves.
Tanking Slurry is a cement based paint you mix in a bucket, lol. It dries light grey. It has the advantage that it sticks to wet walls, is waterproof, repairs cracks, and is breathable, and is also cheap! But it's god awful hard work to apply and the walls must be free of paint. Your house ends up looking like a Nazi gun emplacement but hey, look at all the advantages! Eventually of course you would paint it with any masonry paint. It's a cheap solution.
Another answer for the outside would be to have the rendering chopped off and replaced but that's expensive. A final option would be to have the front of the house partially or full clad with upvc. That makes it waterproof but allows it to dry out because there would be a gap between the cladding and the front of the house. You could DIY that though, really easy. You just need to research it. White cladding is cheap.
I think what I would do is paint the interior and exterior and wait and see. If you still get signs of camp then consider something more drastic like exterior cladding. I think it will be fine though.