Richard, Marty's answers are based on many years of experience. I consider him a 'go to guy' on all he answered.
I will say that my 'roster' contains mostly MTH engines, and I have no thoughts of selling any of it. In fact I just bought a second hand-held controller and a couple thumbwheels and cradles. I tried the WiFi and while I think it is a great product and is the future in operating trains, it didn't work well for me because of manual dexterity issues. In a nutshell, my fingers shake enough that I was always hitting the wrong thing. 
The firm pushing of physical buttons works better for me. I only brought it up to show I am not selling any MTH; it will be around for a long while. Yes, if a PS2/3 board dies in an engine, then I will replace it with a TMCC compatible ERR board. Each person has to make his own decision there.
Your plan looks like it will provide a lot of train action!! Others here are better suited than me to pick it apart for even better operation.
I think 14 awg stranded bus with 16 or 18 awg feeders should work well. I am not using 12 awg on my 11x8 layout with a walk-in to the center.
Separating loops into blocks will aid in finding short circuits in the future, but it works with them all tied together as I have my layout temporarily connected to test got bad trackwork with a variety of engines.
I am aware of it, but am not up on wiring accessories from insulated tracks. I came from an HO background only delving into 3-rail 8 years ago and am currently building my first 3-rail layout.
I'm looking forward to hearing from others and seeing how your layout developes. I always learn from these conversations, and find I can add to the discussion as the project progresses.
