Here we go again - don't shoot the newbie!

All,
This is me. After sitting in the attic for 30 years, I have resurrected my old O(O-27) Lionel trains. I have built my 5 x 8 train table (picture attached) and drafted a layout.

My vision is to have these two loops with the back one (closer to the wall) slightly raised above to have two different levels (represented by the 2x4). In the far right corner there will be a mountain/tunnel and then both tracks will be at ground level at the front.

Here are my initial questions.
1. The layout seems to work mostly. I will need to buy some smaller pieces of track (either at a train show or on ebay) so I don't have to pigeonhole/cram pieces in. When I designed this on AllTracks I couldn't make it all connect. When I measured on the software it appeared that I was off by about 1 inch. Can't I assume that I can angle these tracks and make this work with such a small gap or distance? How forgiving is the track?

2. Once I have the layout blessed and feasible (that's your job). I need to build the grade, ballast and mountains/tunnel. Does anyone have suggestions as to what order i do that? I assume it is:
a. build the grades (either with partial sets of Woodland Scenic to get to 2-3 inches above the other track)
b. Build the tunnels and mountains.
c. Ballast the entire track.

That's enough for now. Thoughts?
Thanks!