Your functions all work together in tandem. At any point in time, you're using Ni+Fe+Ti+Se to understand the world around you. You don't rely on Ti to the exclusion of Fe and Ni.
You seem to be suggesting here that there's no conflict between opposing attitudes of judging and perceiving (Ti/Fe, Fi/Te) when really what they're doing is
switching back and forth between each other.
Naturally the ideal is for the functions to work together over the
long run (or to be more precise, Fe, Ti and Se all work together as Ni sees fit for them to), but to be using them
all consciously and
simultaneously in day-to-day living? The point of the theory is that Fe and Ti
are operating at the exclusion of one another on the micro-level, it's just that they can switch back and forth between one another in such a way that it might
seem like they're happening at precisely the same time. But that's neither viable nor advisable w/r to optimal conscious-attention distribution. It'd be ineffective and inefficient to use both Ti and Fe at precisely the same point in time.
You will always use Ni+ Fe, even when you're engaging in Ti-like activities because, as an INFJ, that is your unconscious preference. Everything you do will always be filtered through your function order. The way you use Ti will never be the same as the way INTP's or ESTP's use Ti because the way you learn it and use it will be dependent on your internal function order.
Again, Ti and Fe are exclusive of one another. When you're using one brain process (see Nardi, esp. on the INTP's "dissociated [Ti] state" upon meeting new people), the other is essentially shutting off.
I find it very difficult, if not downright impossible, to use Fe at
precisely the same moments when I'm exercising Ti, especially considering one is more conscious and the other
isn't. What you're saying in essence is that you can use both optimally
and consciously at the same time, even though for the INFJ Fe is more conscious and Ti is not, but conscious/unconscious processing doesn't work like that. Use one and you're necessarily neglecting (i.e.
suppressing) the other. There "isn't enough room" at any one point for them both to be operating within the same realm of awareness, or on the
same level of it.
Still, you can sharpen your sensitivity to Ti by doing logical puzzles, by dismantling parts (conceptually or physically) and seeing how they work, studying etymology, grammar, dismantling the logic of someone's thinking (perfect Ni+Fe+Ti activity), etc. However, you don't need to try and shut down/suppress processes that you think are related to Fe in order to focus exclusively on learning Ti. Accept the insights that Ni+Fe offer you; when you let everything develop together, you actually gain a healthier and more rounded, reasonable perspective.
Ni-Fe have
led me to the understanding that I need for Ti to be stronger/within my more conscious control, in the sense that I realize I'll need to learn to switch rapidly (i.e. automatically, at my will) between Fe and Ti if I'm to be as effective in accomplishing my Ni(-Fe) goals as I possibly can be. So while I'm accepting the insights Ni-Fe have given me, it's still not the case that Fe and Ti can work at the same time. But I understand there is a time and place for each, such that over the
long run they'll be "working together", or rather toward the same end result.
As far as the theory goes, when you psychologically "exhaust" (or attempt to suppress) your top functions, you end up flipping into shadow mode because you end up relying on your inferior almost exclusively. As an INFJ, technically that would be an unhealthy ESTP -- over-indulgent, brash, hyper-critical and lacking focus... which is pretty much the opposite of what you're probably aiming to do with the development of Ti.
Ti (and Se) aren't shadow functions--those would be Ne, Fi, Te, and Si. The idea of the four-function model is that, for the INFJ, Ni-Fe-Ti-Se are the functions most within our control over a
healthy lifetime. The above mentioned shadow functions emerge when even
those four "default functions" aren't handling a current problem or circumstance properly.
Ti and Se development are stressful for the INFJ because they
require more conscious effort to exert. That is, they require more conscious
space to operate within and, to the extent that they're less consciously-differentiated, will eat up more of the INFJ's awareness and energy. This is what I mean by the tertiary-phase INFJ suppressing Fe--in order to strengthen their less-conscious Ti, they need to devote more
active effort to it, meaning there's less energy left for Fe to use at the time.