Posse,
You not going to like this but it's better you find out before you begin. Many of us have many years of experience building layouts and have been all through these types of things before so please don't take offense as we are only trying to prevent more frustration down the road when your trying to build your layout. We all want you or anyone else to have a positive experience from the beginning.
On your new revised plan I counted 26 grid lines from edge to edge, I multiplied that
x1.5 and came up with 39" overall for the layout width as drawn. At best that is only going to allow you a single 180* turn at an 18" radius or 36" from the outside of the far rail to the outside of the near inside rail which gives you the exact centerline deminsion and you have about 1" to add to that measurement yet. So a single l80* loup of HO track will take up 37" and that doesn't account for the added width of any road bed if your planning on using any? On a small backwood logging/mining line you can probably get away with out any as I will on certain areas of my layout. Oh I'm using 20" radiuses for my old time HO equipment so we are comming close to running into some of the same problems.
So if a single 180* loop is going to take up all that space and your plan is only 39" overall width if the grid lines are 1.5" how in the world are you planning on fitting in all the other tracks and switches. You'd have to use 15" or less radiuses and as you said your loco's probably won't handle it?? Of course you could go to small 0-4-0 equipment and short cars to handle very tight radiuses.
Also as far as the track spacing, I laid out two old lengths of track I had paralell and measured 2 inches from the outside of the far rail to the outside of the near inside rail and placed two of my Old Time 36' cars on them and while they didn't touch, they are an inch and 3/8 wide I'd almost guarantee your loco cabs definitely will as they are at least 1-1/2" wide. Test it out for yourself.
The other thing you'll want to consider is how realistic will your layout turn out if too much is trying to be placed in too small area, not too realistic at all especially for that time period.
Sorry to pour cold water on your hard work but you have to revise your thinking.
Again I'm not trying to be overly critical just helping you to see where your planning needs to be adjusted.