Sorry for you having to deal with this, Virgo. I have been in a similar position with another type, and it's hard when you are not sure whether it is just a different way of thinking that seems incomprehensible but is function related, if it is a typical response of that type when they are unhealthy, or if it is a personal thing and has little to do with type at all. It is all rather discombobulating, especially if that person was very important to you.
Certainly, I've seen a number of threads about INFJs soon after someone has had a serious falling out with one of them. What you are describing doesn't sound typical to me, but to be fair, I don't have a huge experience base of INFJs in real life other than myself and possibly one or two others. They are a bit chameleon like and tend to shift to be whatever the situation calls for, so I find it hard to even identify them, in addition to their rarity. I think what some people are reacting to is that it seems unbelievably common for everyone to have had a bad INFJ ex or friend, to the point where the percentages for INFJ frequency in the population suggest that some of them are likely mistyped. In addition, no one likes being accused of being a certain way that is universally regarded as negative, when the person questioning doesn't know them and seems maybe to be overgeneralizing or including inaccurate information. I think it's true that INFJs do have more trouble than some separating the wheat from the chaff. It's unfortunate, but it happens.
On the other hand, it's also true that INFJs can be somewhat myopic, the more emotionally stirred up they are. They are bad at making their needs known until they are overwhelmed by them. They are good at noticing patterns outwardly or if they are slightly removed, but less good at noticing what's going on with themselves or when they are very directly involved (not just emotional patterns, but behavioural, physical, etc). This isn't withholding, as it's easy for them to know they may be slightly upset, but they have no idea how much till they're at breaking point. And then it's embarrassing to deal with having reacted in a way that horrifies or wounds other people. Also, because conflict uses up tremendous amount of their Ni processing energy, which is terribly slow compared to nearly all other functions (Fi at least is a judging function even though it's introverted), they don't engage in it early on when things are less big of a deal and they also sometimes get overloaded during conflict and just shut down, which feels terribly rejecting to others. Very little details can sway the way they see the whole entire picture when they are in that state too (for good or for bad) and they feel a little like they've lost their own moorings and it's hard to know how to assess anything. The more they are pushed to at the time, the more negative of a reaction they seem to respond with or the more resentful they are to the person trying to push them to communicate.
I think, the healthier the person, the more boundaries they will put on how they express these feelings or on the impact they have on other people. On the other hand, FJs are also notorious for trying to rationalize themselves out of feeling negatively and don't do themselves or anyone else any favours when at last they've stuffed one too many things down or talked themselves out of being upset and the whole mess comes exploding out or else makes them physically ill.
As far as practical advice, I'd suggest leaving the door open to talk but not pushing them before a little time has passed, specifically inviting them to vent (the more room you make for it, the less it will take, and they will be very reluctant to do it voluntarily unless really at breaking point, when it just comes out nastily and in a way they are deeply ashamed of after), and letting them talk till they run out of words. I know there should be onus on them too, but if you are looking strictly from a utilitarian point of view, there it is.
As far as sense of humour, I do think FJs generally give off a very earnest vibe compared to Ne users. They aren't able to think nearly as quickly and so often don't come up with stuff till the moment for it has passed. They also don't have a very physical sense of humour or light silliness about them. However, if you get to know them better, I think there is a surprising amount of playfulness there. They will be very funny at times, but all the conditions have to be right for the moment to come up and often it is the unexpectedness of hearing a certain comment come out of them that makes people laugh. It generally only happens one on one or in a very small trusted group though. As far as appreciating humour though, I think there's lots of that. They are probably pickier about context though, and when upset, will not respond well to people trying to use silliness or humour to lighten the mood.
I agree that in general, we are idealists in a big way. I think the more mature and healthy someone is, the more realistic a view they will take of the world around them and realize that imperfection is a part of life. Their quest for perfection can be a bit exhausting or annoying to a partner, depending on what that partner or friend's priorities are, but in general, I don't think it has to be a big problem. One of the biggest paradigm shifts for most NFJs is realizing that mistakes are a part of life, both for themselves and for others. It is harder to accept other's mistakes when you can't accept your own without shame or guilt etc. It takes time to understand that they will sometimes have to bend for the other person, rather than having the person meet them where they are at. Part of this tendency is not simply selfish though, but rather because they prefer a more rigid set of external circumstances because the unexpected drains huge amount of processing energy from them that then isn't available for every day functioning. So it's an unconscious self-protective measure.
Related to this is that they often won't take chances emotionally unless they feel they've had a specific invitation and they even will take on extra inconvenience emotionally from the other person because they (unconsciously) find it easier to adjust in a voluntary way than have either rejection/upset/adjustment thrust upon them unexpectedly. I believe this results in a rather stilted dynamic because they edit their image so carefully without realizing it, and are always trying to "weigh" less, without being aware of when that is actually distancing others and making the others feel controlled, like they have to hide things, or less than. So a lot of the spewed resentment has little to do with the situation itself and more to do with feeling that the same invitations they've been giving haven't been reciprocated as they should be (which isn't fair, as the other person didn't think it was a matter of keeping track and got no feedback that there was any big problem). This is true of everything from deferring about little preferences, to showing interest in what the other person has to say, to taking on more responsibilities in the relationship, etc. When the INFJ feels like this, they will suddenly become aware of all of the little incidents that weren't big enough to bring up on their own, but now form an overarching pattern and they feel very frustrated, which turns into attacking kind of energy or else an abrupt change of information, rather than adaptation through communication as it should.
Don't know if this gives you any help. I am saying "INFJs", but I realize I am making some broad generalizations that may or may not apply to every INFJ out there. Hopefully though it gives you a little insight as you process all of this.
That's my take on it anyway.