You did well to get everything properly masked as that Diamond paint is very difficult to remove when cured. Also rollering a final coat for the walls is good. Tiling looks spot on.
I think a trial dig near the wall make sense. Don't do it near where you may be keeping the nibs of the existing wall. You may find that the existing wall foundation bears onto very stable base. Try to avoid underpinning.
If there is a smell it is more likely the pipework letting gasses in. An open end on a washing machine connector or something connected the wrong side of the sink trap?
The Velfac and IdealCombi trickle vents are very similar. They are not visible externally. Internally they are metal and flush with the sash when closed and protrude when open.
I have just had gas combi boilers fitted in 3 small new build flats. Much cheaper to do, customers know where they are regarding costs, very little space used, no outside units. I appreciate that ASHPs have their advantages, but they are not always the best solution.
It is a bit tricky as traditionally t&g floorboards were nailed down and there was expansion available between each board, which could be 5mm. If they are glued together the 5mm no longer applies and has to be multiplied by the number of boards which would be too much over 4.5m. Maybe the battens min 45mm will be better.