Looking good! Since you're trying everything out before hand, you might as well try out mixing substrate and getting the right humidity. This will prevent you from having as many surprises when the real deal comes. I had humidity troubles my first time, and ended up using an egg grate on top of some vinyl mesh (can be found at craft stores), which allows for dumping a whole lot of water in there without the eggs coming into contact with it, and without the substrate sinking with the adding of fluid. It worked really well.
Another tip for humidity would be press-n-seal ontop of your tubs, under the lids. Just make sure to air it out regularly. :)
Also my 2cents on putting bottles in the 'bator. I think it is a really good idea, not so much for getting back warmth after opening it up, but for if the power goes out. In the middle of my incubation period this year, a bad storm hit, and I lost power for 4 days. I was able to hook it up to run with my car until we got a generator..but the temp had barely dipped between when the power went out and when I noticed that it went out. Chemically, water will retain heat longer than most other substances will. It's just another safeguard, and imo it is worth it.
Good luck on your efforts! :gj: