1) Angst/Guilt: A month after we started dating, we fell into a conversation about "us." He told me that he cared about me but felt "guilt" and "angst" over us. ("I care, of course I care...it's important to me that you know that I do care about you, even though I cannot be a constant for you right now, and that is something that causes me angst and guilt."). This completely baffled me. As an ENFP, I cannot imagine spending time with a person, showering him with affection, being playful/whimsical/physical while simultaneously harboring angst and guilt towards them. I don't do well with emotional ambiguity. Is this something salient to INTx's? (He's borderline J/P but dominant J);
The one thing that stresses NTs the most, are "emotional equations". Normally the thing a TP is the best at, is problem solving. You issue a concrete problem and get a concrete solution. With emotions tho, there aint always concrete solutions and furthermore especially Fi emotions aren't always formulated as a problem, but rather as a snapshot of the moment which the Fi person wanted to share but didnt want to be solved anyhow.
TPs are very empathetic. That means for them they are rather disclosed towards the rest of the world and keep people behind their fortress at the distance. This is pure self preservation. But if they hold someone dear and let them into the fortress, every small emotion they get from their partner, they make their own and feel overwhelmed by it. So for a successful relationship you need to bring TPs slowly to your emotions. For that Fi's have to recognize first which emotions do feel negative to a underdeveloped Fe user. Thats best done by asking, how things you just told the TP made him feel. Dont expect much, TPs have to learn to understand their emotions, before they can really talk about them, but in time they get better, cause they are the most eager to learn. Talking and asking him about it, helps the most; but give him time to answer, its like differential dynamics to him, most emotional questions TPs cant answer at first.
The problem you talked about in your question 1 directly relates to question 2, therefore I will answer it there:
2) Emotional Ambiguity: He recently initiated another talk about our relationship, which surprised me. In this discussion we both agreed that we're both still healing, but also affirmed that we ultimately want a committed/live-in relationship with someone one day. He then told me that there is always a certain degree of "gray area" in most relationships before each person determines how they really feel about the other. I simply cannot relate: I'm black/white with my feelings! I don't casually like someone: when I like someone, I'm affectionate, demonstrative, loving, adoring. (Hopefully not overbearing (my INTx is teaching me not to be). If I'm ambiguous about them I don't pursue anything with them. Life is too short to waste my time. At any rate, the stereotype about ENFPs being overly enthusiastic puppy dogs is in my case quite true). My question to you is: what does it take to get an INTx out of the gray area?
Most simple answer for me here is that he loves you very much. Indeed he loves you so much that he is afraid to loose you. His fear is a direct result from your black and white approach. Tho the black and white approach is one of great loyality and TPs like that, it does at the same time build up a certain tension. Feeling inadequate regarding ones own emotional responses, the TP can start to think as if he wasnt doing enough for the relationship to keep the girl. This especially seems awkward if he infact aint doing enough.
The only solution I have found for it is that the NF needs to learn about grey zones, especially in regards to fundamentals. A TP likes to see things that come a long way, like for examplke a relationship that lasted 8 yrs, in comparison to one that lasted only 1 week, as more safe and stable. This aint natural to NFs, if they have mood swings they can bombard the lobngest relantionship.
The other thing is that the TP needs to learn to open his feeling side and to start to trust the NF. For that tho he needs to be aware of his own fears more clearly.
3) Random Intervals/Lack of Pre-Planning: I happen to know that this man is a romantic, at least in previous serious relationships. When he wants to see me, however, he generally only asks me out 1-2 days in advance. I realize it's my job to teach him how to treat me, and ask for more advance notice, but I get hurt each time as it seems disrespectful and indicative of a certain lack of regard/interest (which contradicts his affectionate actions when we're together). Any advice about his lack of pre-planning? Is this an INTx trait or does he lack the courage to tell me he just doesn't feel strongly enough about me to pre-plan?
That's an unfair thing TPs do. Infact most have an attention span of like 3 meters, which means "what aint there right now, they dont think of". He not thinking of you tho doesnt mean you aint in his heart. That becomes more clear if you were to dumb him. After you did that he'll start to heavily feel what he is missing. It's an unfair thing TPs do. It got better for me, when I moved in with my girl. Now she can sit in my lap when I am working on an electrical circuit.
All this information is based on my relationship to an infp. Many things needed a lot of time to develop between us but now after almost 9 yrs, we have come a long way. Its the fundamental difference between us that has brought us great fights, but great insights intpo the other and ourselves. It's a thing TPs love and obviously NFs do aswell.
The best advice I can give you is that you need to be able to talk a lot and good quality talk with your TP. If that is possibly, you two have all that you need.