For the Concreat wall....my first thought would be to either get a candy mold of little squares and just mix grey with white chocolate and mold all the bricks. If thats not possible I would do form them out of Fondant or White Chocolate Modeling paste colored with Dove Grey Petal dust (this will help give it an inconsistant color like cement has). Then when you go to stack your little bricks let them semi dry then use toothpicks or BBQ skewers to hold them vertical. If you assemble them upside down then you'll have the extra wood to stick into the cake when you put it in, you can also create a nice curve or a soft
S" shape doing it this way
If you want to make a Wroght Iron R.I. fence I'd say start with supports, a heavy gauge wire and pipe the royal icing over it for the sides then maybe pipe all the stuff in between, it will still be very delicate so make a few. Or you could use the wire then cut a square of fondant and pipe it all onto that so it's less likely to break.
For the mini's I would use Modeling Chocolate, these will make your eyes hurt after doing so many so start with enough time so you can rest your eyes. For Pots I'd just form little balls of the Chocolate then work them into the shape of pots, do some kinda spanish and round and others can be made in that "V" shape.
For a tree I would use gumpaste or Modeling Chocolate that has an internal bbq skewer then wrap heavy gauge wire around em and use superglue and floral tape to give it support. Once you have your support set you can roll strips of your product and just attach them to the tree stucture and use modeling tools to smooth the junctures but still have that rough root like look (see pic attached). Then you can pipe green royal icing or marshmellow to make leaves. I wouldnt use the sugar gun just cuz you'd want that hand rolled inconsistancy to give it a natural look.
To make the man I'd go to micheals and pick up those Silicon face molds used for Clay making. Maybe you can cheat on the chair and use one from a doll house? Then mold him onto it with his easel.
I hope I posted the pic right, sorry if it does it twice, I'm new to attaching.