Belt and braces job this one soot is hygroscopic (I think that's the word) it keeps sucking up water in the atmosphere and reactivating. Just skimming will reactivate it. Maybe a hit and miss vent in the chimney if it is redundant. Salt inhibitor etc.
@Josh76 sounds like a bit of a nob to me. Probably been an insufferable fusspot throughout. The tradesman told you he could not guarantee movement on the staircase there is no need for a second opinion from others who are not doing the work, never seen the work. Bunch of fishwifes the lot of you!
Overthinking it ... Strip the wall ... Cut the laminate back circular saw chalk line multi tool for awkward bits ... Stick the boards on foam the gaps chuck a few 100mm pbscrews in red plugs ... Grip fill the skirting back ... Skim away
All good advice but difficult to give advice. The best person to ask would be a local builder familiar with your particular housing stock. Perhaps the cavity is full of debris perhaps that portion of the wall is a cold spot for condensation perhaps the window wasn't fitted correctly ... It could...