i've had a lot of differing experiences with infps and i'm intrigued by what you're saying. the "feel safe to be close" thing rings true, the first girl i ever got together with was an infp and she found me extremely safe, comforting, warm, but she could never see me past the end of her nose. it was frustrating.
the way in which infps want to be understood feels different from the way i want to be understood. they want to have their subjective feelings grasped by others, i can't tell if there is any other way than similarly being a dominant introverted feeler and having the same values across the board. sharing the same experiences and being able to relate those effortlessly. i feel like part of the issue for me was that there was no faith in articulation or communication, you either had the same subjective experiences/feelings or you didn't. whereas i rely on expression to help me locate and imagine the inner inside aspect of things outside of me.
i think part of it is my desire to build bridges and dialogue and kind of direct those efforts. that i want us to connect our expressions, to produce themes and motifs, to find musical ideas that can represent and symbolize things. whereas they often have an inner reading of whether something is hitting the right way or isnt, if this person already knows what they mean and is wearing the feeling or isn't. i don't know if this makes any sense, but they get frustrated when others don't have all of the details that are so extremely subjective already mapped out exactly as they do, aren't experiencing the same subjective state simultaneously, etc.
i am still struggling so hard to understand Fi/infp devotion. i am in a relationship that involves Fi devotion. i am the one for her. one of her "causes" as you say. for some reason my image to her is heightened and more significant than those of others, stirs more powerful memories and emotional resonances, etc. and maybe i'm making too big of a deal of something or am just inexperienced or am just too much of a worrying thing, but this feels like a totally different way of operating than my Fe does for me. which is so ephemeral and fleeting and momentarily based.
Hmm......I don't quite relate to the description of your INFP ex. I definitely want my subjective views to be grasped, appreciated, respected and ideally agreed with, but I don't expect that to come out of nowhere. It can be refreshing when someone simply "gets" you without need to give exhausting explanations, but it would also be boring to have nothing to explore about each other.
Also, I can get profound impressions about people, but I don't always trust them anymore, because they have been wrong. Maybe an immature INFP trusts that instinct as if it is faultless, and that is why your friend didn't see the need to actually confirm her feelings. For me, it is exciting to get that confirmation. It cements the feelings that drew me to the person, or it pleasantly reveals a side of them I did not expect.
It seems that may be what you are looking for, and I agree with you on that - the need to build a solid bridge where the INFP has cast an imaginary rainbow

.
When I was younger I would sense something about someone and make that impression & that was that, but after having several people I was dead-set against turn out to be close friends, I've learned that I am just plain off sometimes & I needed to give people a chance to reveal themselves to me. I would also do the opposite....feel a connection & be sure that person was "just like me" and then be sorely disappointed when they were not (and I'm not looking for someone "just like me" anymore either). Part of it was letting my imagination get carried away and creating a person who didn't really exist in place of that individual. Maybe your friend had a person created for you, and you didn't really fit that. So she felt she grasped you, but it was only the you she had built in her head. INFPs will have discussions with people in our heads, and then we feel all is settled, but we never pursue that discussion in reality, so it's not settled.
But that's another tangent....
Anyway, because INFPs are sooo sensitive to criticism & conflict, we may avoid discussing our deep feelings for fear it may result in such. I enjoy discussing my views and expressing my feelings otherwise. Again, it can be a safety/closeness issue. I admit when someone brings up a topic that touches on my very deeply held beliefs, I get anxiety just thinking about discussing it.
I also hate to have my imagination bubble burst about someone when I suspected they may feel the same & they don't. Coming to terms with bubble bursting can be hard for an INFP. The bubble is like some warm womb we never want to leave, but we have to if we want to live our lives in the world. Maybe some INFPs you know have not been ready to leave that bubble yet, and your J makes you need to keep in touch with reality so that is frustrating.
We can also get very INTENSE in discussing something seemingly casual, such as music taste, and then people get weirded out. Those experiences make us clam up more, but people don't understand why we'd be so hesitant to talk about things we're passionate about (and really dying to discuss).
Not sure if this is typical for an INFP or if this is what you are getting at, but I will like to cover every aspect of an issue into the minutest details. I like to quote lyrics, books, use metaphors, and reference images to explain my thoughts, since Fi is difficult to explain in everyday language (on a side note, I suspect that is part of why many of us INFPs use some artistic outlet to express ourselves). Doing that does help me to see that someone grasps me & I grasp them, because there's an outside expression we both relate to.
That was a lot of rambling...hope some of it makes sense, haha.