Regarding when the advantages of incarnations end. This is a variable time, determined by many factors. Incarnations gain the entity an acute sense of mission. Think of your current density, the 3rd, and your current mission, which is to determine your orientation, Service-to-Self or Service-to-Others. How would a spirit form so decide? What sacrifice is required for Service-to-Others? As you say, easier said than done. The proof is in the pudding. Put your money where your mouth is. In other words, when push comes to shove, the orientation decision of the entity is proved out by life circumstances. In some cases an entity continues to learn through incarnation, even when in a higher density. Incarnations are a learning tool.