INTP function orders under various schemes:
You have to state which scheme you are using for "tertiary" to mean anything as the order differs in each. For example, if you are talking about Thomson, I agree Fi is the way to develop the feeling function for INTPs and to reduce the potency of the inferior. If you are talking about Jung, Se is definitely a great stress-reliever for me, plus it pulls me out of myself to engage with the world on its terms, rather than mine, which is exactly what I need. But if you are talking about Beebe, (as you appear to be by stating the tertiary attitude mirrors the dom, despite also claiming Beebe is wrong...), then no, Si doesn't help develop a more rounded personality for INTPs and I don't know anyone who subscribes to this view.
The quote you used is misapplied. It doesn't refer to the tertiary function but to a "third way" in conflict resolution. This third way is more likely to be mediated by the auxiliary function (dom=thesis, inf=antithesis, aux=synthesis) than anything else (in Jung's thought too - as indicated by my previous quote.)
It's not that I don't like using it - it's that I can't use it. If anything, IT uses me. Or it used to, before I became conscious of it. Now I just recognise what's going on and get over it.
You too are mixing your models / theories up. One of the reasons it's difficult to have a conversation about this stuff is because no one can agree on what goes where. But in my view, it doesn't make sense to claim all of your inner desires are invested in your least used function (unless there's something very wrong with you, and type doesn't cover pathology). The inferior is the part that feels *least* like you (which is why it gets projected).The way I understand MBTI is that the forth function in your case Fe, in mine Fi, would be the Aspirational Function. The function that "Drives" us. Where we have all of our inner desires.
If you are ENFP, then your association with Fi makes sense, because it's your aux, not your inf.Sense of purpose...is what I get from this. Yes we have a leading role but I kinda think that we are really directed by our inferior. Almost like everyother function is "serving" it.