I may be able to shed some light on the development of Fe. I am at a young age, twenty, and am only just now coming around to the use of Fe, and the changes that it can cause are quite fresh in my mind.
I feel like the existence of our tertiary and inferior functions are perhaps always there, only we do not know how to use them properly until we become mature enough to understand what they are useful for. I was always able to use Fe, though when I was younger; it oriented itself in the way of allowing me to understand people's feelings easily by their faces, reading into the emotions of situations, and the manipulation of people by way of their mood or emotions. I could relate emotions to the context of situations (Ne-Fe usage together?), and understand instinctively the intricacies of situations with the added dynamic of emotion at play. IE:You see a teacher and a student talking. You can understand the student-teacher relationship, you understand the nature of people talking, and you read into the way they are talking to each other, the words they are saying, etc, to draw some larger idea or understanding from what they are doing. It is nothing more than a support for the highly developed an Ne, and to a smaller extent, Ti, both functions that the ENTP see as useful, whereas Fe is just some thing they use when they need it.
But, I didn't care as much as I do now for the emotions of other people, and so Fe was of little use to me. There is no reason to develop it, or use it well, when you don't understand its use in the first place. So, Fe was always perverse. It was a very rough usage of it. And all the mishaps that its rough usage will bring about usually boil down to a lacking understanding of emotion, and therefore not caring for its value. As easy as it is for ENTPs to understand the basic nature of people, how people's emotions are brought out, and how certain roles or instances can bring them out differently, when our Fe is weak we seem to have a complete and ironic inability to understand how what we do emotionally affects people.
Thus, in my experience the ENTP can be very fickle in nature with people. One day we will chat their ear off, and the next day be totally absent-minded, forgetting to even talk to them and making them feel as if they did something wrong. A perfect example would be my dating life. I went through so many girlfriends during my teenage years, after having some awesome conversation with some random girl I had met, then obsessing over her and playing her emotions, situations, and the larger context of our dynamic until I was in a relationship I wanted with her. I felt she was the most interesting thing on earth, only to become bored after I thought I had figured everything there was to know about her. My lack of insight into how this affected her is largely due to a perverse use of Fe. And while I and other ENTPs likely do not mean to be jerks, it simply happens. Our Fe is not strong enough for us to take the proper precautions so that people do not get hurt, we simply re-rationalize our behaviors constantly to make what we did sounds okay to anyone else and ourselves, convincing otherwise anyone that tells us that we are jerks. I likely unwittingly used Fe very strongly when I wanted the girl and was in the wooing stages, but after I had her, I forgot what Fe was good for and made up some way to be rid of her Another likely scenario for the lack of Fe is when an ENTP pushes peoples buttons, teases, and ends up unknowingly hurting them, because we simply don’t understand why those emotions matter. We understand when not to do things in certain situations, but we do them anyway, not being able to stop ourselves, simply not caring about the affect our actions have on anyone else.
I believe strongly the tertiary functions are always there, and Fe is always existent to the ENTP. But, the trick to bring the tertiary function out is to understand why it is important, and what it is good for. This likely only comes with maturity. Especially something like Fe. Part of growing up is not being a jerk anymore and caring about others. I suppose the process could be expedited if the ENTP used their other functions to realize what positive affect the development of Fe would have on them, and why it is logical to develop it. The trick though, is largely in actually making the effort to care or use it.
I think it likely also leads the ENTP into situations where they use Fe to manipulate people (like me, with the girlfriend example. I put them in a relationship I had little intention of staying in, even if I might have felt like I wanted that relationship very much at the time). In such situations, I worked almost entirely off small things that I would hear from whomever I was wooing, piece them together to fit form some larger picture of the person, and then used Fe to work my external agenda. I talked with the person emotionally, with only the intent of seeing if I could date her, reaching my goal. The problem being that my agenda could be changed on a whim, and when I was done with her, I was done with her, forgetting Fe and how I used it to understand emotion.
A lack of Fe basically means that a person does not understand what Fe is good for. As soon as an ENTP realizes its value, the ENTP will learn to develop it. Only when the ENTP realizes the values of emotions, or at least the importance that other people put on them, will they be able to use Fe wisely. The ENTP will likely never care for emotion and feeling as much as the ENFJ or the ESFJ, both feeling dominants. However, we will be able to at the very least understand the value that others place on it. And in that sense, we have succeeded, because putting on the façade of caring or at least caring enough not to take precautions not to be hurtful is accomplishment enough in removing many of the most negative traits a weak Fe will cause. No longer will the ENTP say the wrong things to people, manipulate, or be fickle. It gives the ENTP a type of sensor to gauge the general ambience of situations or people, and understand instinctively what not to do wrong. It allows them to be warmer towards people, smile and be friendly in conversation, instead of just the general intelligent dialogue, random ideas, and questioning that Ne tends to give. While never particularly poor socially, smooth and funny enough to get by already, ENTPs now also have their warmness behind them in the social arena. They also will start to become much more loyal.
Beyond that, they no longer depend so much on Ti as a lackey for Ne. Ti is much more inwardly focused, while Fe is outwardly focused, so by understanding the importance of Fe, the ENTP begins to focus more on people and become much less socially awkward. Fe allows them an alternative to Ti.
When the ENTP realizes what use highly a highly developed sense of emotion is good for, and intensely develops Fe, they start to appear almost as if they are a feeling dominant, and that is when the ENTP can develop truly unique and highly valuable gifts. Now, Fe is no longer merely something to keep the negative from happening, as much as it is something that will make things better for the ENTP and the world they live in. A perfect example of an ENTP using the Ne-Fe dynamic quite well is Barack Obama. He seems to be a feeling dominant, but if you look closely at his speeches or read his book, he is full of extraverted intuition. And the man refuses to make a move without intensely calculating the dynamics of the situation first; resembling introverted thinking (he manipulated health care variables for a year!). An ENTP with intensely developed Fe can be quite charming, pulling on the emotions of people to reach an external agenda that is often quite positive. We can make intuitive perceptions to the people around us, when thinking in terms of people more than ideas, understanding their talents and potential, being catalysts towards positive change. We can easily get people excited about the reformation of a system, or new ideas. We are charmers and manipulators, but never for a negative reason or to merely satisfy our Ne curiosity.
An extraverted intuitive with highly developed Fe (can only be the ENTP), is spoken about by Jung in the following:
“It goes without saying that such a type is uncommonly important both economically and culturally. If his intentions are good, i.e., if his attitude is not too egocentric, he can render exceptional service as the initiator or promoter of new enterprises. He is the natural champion of all minorities with a future. Because he is able, when oriented more to people than to things, to make an intuitive diagnosis of their abilities and potentialities, he can also 'make' men. His capacity to inspire courage or to kindle enthusiasm for anything new is unrivalled, although he may already have dropped it by the morrow. The stronger his intuition, the more his ego becomes fused with all the possibilities he envisions. He brings his vision to life, he presents it convincingly and with dramatic fire, he embodies it, so to speak. But this is not play-acting, it is a kind of fate.”