1) After you create the toolpath, a small Accept Toolpath window opens. Onthe bottom of this window, is a "Save Toolpath" button, which is where you save the g-code.
2) You must do a roughing pass before a finishing pass if you use a short tool. If the parameters for the tool are entered correctly, then MeshCAM will prevent the shank from crashing.
3)Yes, I believe it's in seconds.
4)For wood, using a 3mm ball endmill, I usually cut at 2500mm/min or faster, at about 14,000 rpm. I've never used MeshCAM for cutting aluminum.