You'll be fine with pea gravel in the mix, it will move and level easier as the aggregate is smaller. It will be a weaker mix but as it's a base for a patio it'll be fine.
It is not an EPS insulated floor like Tetris it'll just be either thermalite type blocks or light concrete.
530mm is the size of a closer block.
If you have AAC blocks your floor will have a u value of around 0.6-0.5
Each to your their own James, but that addon you linked to is not really in keeping with any elements around it, it has a slate roof etc it just looks stuck on. But if that's what you are going for....
I am going to presume those stars are replacing the word 'standard'.
How long have you let it sit for to dry out? With the weather it should not take long.
Make sure it's water causing the damp and not a stain on the brick work.
For the outer skin to become saturated from an internal leak is unusual.
They are Durox blocks but any aircrete block will do but you'd...
To direct run off into the drain?
But I agree it's been placed there to trip you up everytime.
Are there any services entering into the building at this point?
Have you tried pressure washing it since you already have one?
You could shot blast it or use paint stripper or use a floor buffer with a mesh sanding pad or just use a floor scraper and lots of time or a wire brush on an angle grinder.
If you grind it may be better off buying your own tooling...
If it is just condensation then that one step will solve your problem.
Might be worth getting a cheap hygrometer and keeping a log over a month.
One other thing is to check the render at the bottom where it meets the exposed brick work. I have seen quite a few houses where the water catches the...
Have you thought about knocking it down and starting again.
New roof, new walls, your newly there already.
But seriously it might be cheaper to start again.