Awesome work Darwin! I love the detail you've put into this.
One way to mask to get a soft demarcation line and prevent overspray would be to do the following.
First off, doing this technique would mean applying your two-tone pattern in reverse.
First, take a piece of paper cut into the shape of the darker pattern. Tape a rolled up piece of tape (or two or three) on one side of the paper. make sure it is rolled with the sticky part outside, as if you were making a single-adhesive-side tape into a double-sided piece. Tape it then to the model. The doubled up tape will hold the paper piece slightly off the model allowing the AB to make a soft line. When you AB, point the AB straight towards the model, but angle it slightly away from the center of the paper that is taped to the model. For painting the demarcation line between the upper and lower fuselage, get a piece of paper that wraps around the top of the model. Hopefully this should help.
Also, great idea for finiding the center of gravity on the model. What I usually do is put in the nose weight, then tape the two fuselage halves together, tepmorarily put on the tail surfaces (if they fit w/o glue), and then dry fit the wings (again, if they fit w/o glue). Then I hold the model up with one finger in the one rear gear well (where the gear gets glued in) and my thumb in the other one. That usually tells me whether I need more weight or have enough.
Again, outstanding work on that model Darwin!