Just leave it…….normal rules around pressure treated wood is that you dont treat it until it starts to grey and the naturals oils have run out. Otherwise the paint or stains just peel off with the oils
Have you thought about building the pergola ontop of the deck, rather than incorporating within the deck structure, just in case you may want to take it down in the future???
You let them finish the job. You got a 10 year backed insurance guarantee to fall back on, if there are problems in the future. Because they are not going to dig that stuff back out and replace now.
Weeds are the risk, the sand and weed barrier would block out all the sun to the soil below and stop weeds germinating. Also when mud gets wet its all squwelchy, the sand will help take water away from the surface and stay nice and firm.
I would put a vent top of side wall.....and a vent low down, so cool air comes in at the bottom and pushes the warm air out at the top. The fan will help that happen
So the heavy rain showed up some drainage issues. Found out there was another path under the path i also took out. More digging and removal and another 1.5t of topsoil and its now pretty level along with 2.5kgs of grass seed
I use bolt down post holders without any issues for the past 5yrs........just make sure you use pretty good long anchor bolts and also fill with resin too. Nobody be pulling them out in a hurry