For me, my boundary issues have more to do with:
1) Needing gut feelings to be proven.Sometimes I have felt something is not right, but don't trust that feeling until I can articulate why and have proven evidence. Being fair matters a lot to me. As I've gotten older, I've realized that if I feel that way, that feeling is rarely wrong and is there to protect me. Sometimes we subconsciously pick up on physical and emotional signals before our conscious mind catches up with what we are seeing. Now I go with it before I've received confirmation.
2) How I develop relationships and process information.Usually I start from a closed position and over time, with repeated interactions and observation and seeing what people do with less important pieces of information, I slowly form an opinion of someone's expertise, trustability, merit of ideas and so on. It takes time for this to happen. Once someone is in, they kind of have the run of my inner world, as it is quite a process. My focus is prevention and extrapolating what I can expect from the person in the future.
I think with NFPs, they start from the opposite end. Everyone is given a chance, until proven that they don't deserve it. This leaves them more open to new ideas, but also more vulnerable to being hurt.
Where my system breaks down is that sometimes the people who are in my inner circle change, show unexpected character flaws, or lie. It takes some time to really believe that someone I've grown to trust implicitly would do that to me and so I give them the benefit of the doubt while trying to figure out why I'm feeling emotional noise that unsettles me. Sometimes I'm not sure if the breakdown in communication/trust has to do with them or me, and so I have to exhaust all the possibilities on my part before accusing them of being responsible. That allows a person to have great influence (and potentially do great good or great damage, depending on who they are) with me. Because I rely on Fe for navigation and for exploring my internal world through external people, their perceptions of me can really skew my own perceptions of myself.
For this reason, I'm not going to accept just anyone's assertion that I am paranoid, selfish, unremorseful, etc, especially if they don't know me well. That would be giving a totally unknown person power to influence when I'm not at all sure whether they are trustworthy or accurate in their perceptions. Instead, I'll file what they say away, try to figure out their intentions, check it against other feedback I have gotten, consult my own feelings about it, and if I still disagree, save it to revisit because there must be some reason why they have arrived at that conclusion and I'd like to understand what it is. Maybe it's a different perspective that we both have that renders my behaviour incomprehensible. Maybe they are going through bad stuff right then in their life that colours their perspective. Maybe I just can't see the whole picture yet and will later conclude that they were right.
3) Ability to shift perspectives - I think that one of my strengths is the ability to shift perspectives easily and look at a person or situation from many different sides. This makes me less judgemental in some ways, but can also result in me losing track of what my own perspective even is. I honestly don't know sometimes which is the most accurate, useful or valid standpoint at the time because I'm able to bend things to look a myriad of different ways. Therefore, I need to rely on articulating what I see to someone outside my head and have them clarify by asking questions and reacting so that I can decide which perspectives merit more weight. For this reason, I need more than one reliable person whom I can trust, so that I have several sets of information/feedback to compare. You can also imagine why this information gathering and comparing process doesn't happen in real time and can be pretty lengthy.
4) Distrust of emotions To me, my emotions are pretty changeable and may depend on which perspective I'm looking from that moment, how I feel, how many other stressors are in my life, how tired I am, and so on. Therefore, they are no kind of guiding light for me, other than indicating that there is something there that needs some attention as soon as I can get to it. Because emotion actually jumbles up my signals, I need enough distance that I can discuss things in impersonal theoretical terms with myself, rather than personal emotional ones. That also means that when people bring strong emotions to a discussion, it mixes up my signals and makes it difficult for me to process the information quickly. I need time to cool them off into something that is more theoretical and impersonal.
5) Weird and unhealthy people like me Because I'm slow to express judgement and initially seem pretty malleable, I think a lot of people assume that I'm more sympathetic to them than I actually feel. I think I seem kind of open and in combination with not trusting the gut feeling thing, that used to be a problem. Strange people would often pick me out of a crowd to approach and sometimes I felt obliged to give them information that was unreasonable for them to have requested etc. I think a few years of busking really helped me to trust my gut and also to not feel afraid to assert myself more. I realized that if someone does something that is socially out of the ordinary in a negative way, it is then okay to respond in a manner that is also not the usual way you would respond to someone you don't know. I started to see that you can still remain pleasant, while also not leaving yourself open to unreasonable behaviour.
6) I like solving human puzzles I enjoy being around people that are not easy to figure out right away. Over time though, I've realized that part of what makes people hard to figure out is them giving mixed signals. This often is the result of a fear of vulnerability, so they give out conflicting messages. It sometimes takes me awhile to realize that I'm actually dealing with an unhealthy or unreasonable individual. I think at least in the case of romantic relationships, I've learned my lesson on that.
7) No one is all bad Even controlling or insecure people have attractive qualities. The ability to look at situations from multiple points of view is not always a good thing, as it can allow me to let people into the inner chambers of my heart that are not going to be considerate guests there. It is sometimes hard for me to distinguish which behaviours to give the most weight to.
8) Figuring out the why turns my alarm bells off. I'm not saying that I can't ever recognize that certain people should be avoided, even if I can understand the reason for them acting as they do. However, for me, what fills me with the most unsettled unsureness or even terror is simply not knowing what to expect or why someone is acting as they do. Once I figure out the why, then I have information from which to construct a course of appropriate action. As long as I still have courses of action left to try and ameliorate a situation that I'm invested in making work, I will continue trying to exhaust them. Sometimes this is long after I should have distanced myself.
edit: Not sure if any of this answers your question, PB, but hopefully somewhere, somehow it is useful.