Run a taut string wound around 2 bricks of the same size or similar across the distance. A 4 foot level with one end steadied on the high side brick - measure from string to bottom of the other end of the level, multiply that times 3 if 12' distance.
How often do you plan on moving it and why? It's gunna be a pain to keep patching the chimney holes.
Once you put the cement board & tiles down, the plywood isn't going to burn.
"drywall or put up particle board"~Plywood is even better.
I would put 8' fixtures, the number will depend on what kind of work you will be doing and in what part of the shop. You can never have too much light (unless your developing film).
Nice project, worth the headaches! On the house sealing part, I used to install 2" foam insulation and backfill with gravel. The gravel allows quick drainage to the permadrain (if used) or at least keeps saturated dirt away from the foundation.
Looking good. The bricks you have securing that tarp will only cause more damage should the wind turn it into a kite. Best to nail boards on the edge of the tarp covering most of the perimeter.