I feel like you are almost painting a picture of my wife and I as some cold, unfeeling couple. We are more then aware of the condition of emotions and feelings. I know it can and will happen when someone goes into a relationship such as this, but I think it is better to clarify our process step by step.
#1: A well written ad pointing out that indeed that my current relationship is a primary relationship and that my wife and I are a package deal. This usually weeds out 90% of the "applicants" (which is why a true unicorn is so RARE) because everyone has their own, personal definition of polyamory. If the "unicorn" decides to answer the ad they do so accepting these terms.
#2: When my wife and I find said "unicorn" the relationship will always start as just friends. Meaning that we will go out, hang out, get to know each other, chat, text, etc. During this period we would feel out our potential unicorn. It also gives us the chance to gauge compatibility. This has worked wonders for us because it enables us to see if there is more of a connection to one of us then the other. (i.e: we find out the girl is more attracted to my wife over me and visa versa) At that point because it is just "dating" and no committment is made, we can chose not to pursue a more intimate relationship.
#3: If the potential "unicorn" moves on to the next phase it is because she has found a genuine connection to both of us that does not strictly revolve around physical, sexual connection. Yes those things are important but they are not the entire foundation for a relationship.
#4: If the feelings of the potential "unicorn" change in the future; we understand that happens. And if that does happen we will cross that bridge when we come to it. The only comment I will make is that my wife and I are the primary relationship and that means our relationship comes first.
Also... You mentioned something about "Hmmmm... no wonder most unicorns have low self esteem and are merely looking for scraps of attention from established couples, to agree to such a scenario." I find that comment a bit offensive because it almost seems like you are blaming couples like my wife and I for someone's low self esteem. A person with low self esteem should not be going around looking for polyamorous relationships in order to find love to raise their self esteem. Self esteem should be personal and the responsibility of the individual and not the couple. This is why we have our step by step process. If I feel like someone has self esteem issues and they are merely "starved for attention" then we will not proceed further into a relationship.
"I have loved the stars too fondly to be fearful of the night...."