RE: Longitudinal stability analysis
It's a pretty involved question and various people have various methods that work for them. I calculate the stability based on the equations for stability I learned in college. I have calculated CG for various airplanes and various configs, and have never been surprised by the flight characteristics. Others use the tail volume coefficient and compare that to a table of standard values.
The first thing I recommend is that you do a search in the aerodynamics forum for 'stability' and find some of the threads where it's been discussed before. It's been discussed in depth. Read up, then come back here and we'll help on the stuff that might need explanation.