That was a good post FL.
Except, I doubt you can really be thinking about a solid relationship with an average ENFP unless you got into their elite category, which tends to be difficult for an INTP. At least more difficult than with the INFP, simply because INFPs tend to have more in common with them.
As far as I am concerned though, most ENFPs will pretend to like you just to avoid problems and possibly even go so far as to mislead you into believing that you're getting closer to the 'non-disposable'category.
But then again, little would you know, they just start slowly drifting away without you having a clue why. Seems to me, unless they believe that they need you, not just enjoy you, you're marked for replacement unless you somehow manage to hit the 'safe heaven.'
I agree that INFPs are more likely to just blow up on you, yet again, I think its easier for you to work things out with them than with the ENFP. The ENFP, being in the same position would just build a wall between themselves and you and the friendship will begin to evaporate there, whilst the INFP, by blowing up on you will open the door for a confrontation to work things through. This may lead to honesty and clarity, which are essential to the INTP.
So I dont know if the ENFP has a one up on you there by having the ability to forego exploding on you in heated anger. Because what they do is much worse...they are being subtle about the same thing that the INFP is being vocal about..
Let me put it this way…
In my experience ENFPs like to keep their friends close so that they can keep their finger on the pulse of the relationship. In fact, they like their relationships so close that it can be kind of claustrophobic for their friends. Also, they often have a bit of a paranoid streak, and keeping people close reassures them that the relationship is solid. If circumstances create some distance in the friendship, sometimes it can awaken their paranoid streak; and that's when I've seen them grow cold on a relationship and drift away from a friend.
You mentioned that you were separated from your friend for a year. It may be that the separation awakened your friend's paranoia and it's hard for him or her to rebuild the friendship. Also, I've seen a case where an ENFP grew frustrated with an INTP friend who wanted to keep more distance between them than the ENFP preferred. It made the ENFP paranoid when he couldn't charm the INTP into a close relationship, and so he turned kind of cold toward the INTP.
INFPs, on the other hand, aren't as worried by distance. In fact, because they like to idealize relationships in their head, a little distance or a period of separation may actually benefit the relationship in the sense of "making the heart grow fonder." A too-close friendship may rub the INFP's nose in the daily irritations of the relationship and take away some of the luster of the relationship in the INFP's mind. A more distant friendship or even a period apart may benefit the relationship by giving the INFP time to rebuild the idealized version of the relationship in his mind.
So in that sense, an INTP and INFP may be more in synch in terms of surviving a separation or being comfortable with a more distant or off-and-on friendship. That's my own theory, anyway.
But other than that, I wouldn't doubt the quality of an ENFP's friendship. If anything, their friendship can be so strong and close that it becomes a little smothering (at least from my point of view). I think the main thing differentiating the friendship of INFPs and ENFPs (absent of any other problems) is how they react to personal distance and separation. Distance and separation can make an ENFP paranoid and ruin the relationship for them, while distance and separation can actually improve a friendship with an INFP and delay the onset of the stage when INFPs become disillusioned about the relationship and start putting a lot of minuses on the friendship scorecard.
So maybe you want to look at your friendship with your ENFP in that light. Your inability to cross the threshold into the "elite category" may have something to do with that, especially in light of the long separation that occurred recently.
It's true that ENFPs insist on having a lot of control over who becomes a friend and when they let friends into the inner circle. And it's true that they grow disillusioned and move on when a former friend won't play the role that they want. But from my experience it's not because they're ditzes or because they want to use their friends as stepping stones. I tend to see it as governed by their paranoid streak. Conditions for the friendship have to be right, or ENFPs get paranoid and lose interest in the friendship. Much as conditions have to be right so that INFPs can idealize their relationships for best effect. Just my theory, of course.
Note the following link, where ENFPs are linked with a tendency to Paranoid Personality Disorder:
PTypes - Correspondence of PTypes, Keirsey, Enneagram, Psychiatric, and Astrological Types
FL