Unofficial Answer to Possession Rule Questions

If your internal stacking system is just nesting cones without placing them on a mobile goal, then YES, this would count as possessing more than on goal at a time and therefore would be illegal. The definition of “stacked” means that to be “stacked” the cones are on an upright goal. Nested cones not on a goal are not considered stacked. “Stacked” cones are not counted toward the one cone possession limit.
Yes, you may put a mobile goal (your own) inside a concave part of your robot.