Just a quick reply, I have a 5 year old’s birthday to go to. If you need more info let me know and I can give more detail tomorrow.
The z axis is a leverage problem, You are correct. The other issue is long legs also have a similar leverage issue but divided by 4. So the ideal solution is a table that the bottoms drops out of. I am working on making one to show the concept. That way you have short legs as well.
Extruders, you are correct again extra drivers fans and wire. Everything plugs right in. The 30A will work, you need to change the firmware to say 2 extruders instead of 1, literally just a number change.
Yup extra belt for your desired size.
not sure about the 123D, are you talking about the 3d scanning software? If so, yes STL’s will work in estlcam, and other CAM software. Pretty sure octo print will work just fine for cnc, it is basically the same as printing, just sending gcode over.