I've a theory. I think that many people grow up hearing some kind of "promise for the future". Like the one described on the "simple man" song. The song captures a feeling about a "more simple time" that, in practice, never existed. It is an idealized version of the real past where only stories of the "winners" are remembered and the obstacles are already overcomed.
You grow up hearing that if you study hard, you'll get a good job. That the job would be stable and allow for you to buy a house and provide for a family. Of course, the part of finding a loving partner is taken for granted and "obviously" you would be having a family.
The problem is that nothing is simple. You can study hard and still won't get a job. Or get a job and work hard and be fired or "let go" because "the economy" or reason that have nothing to do with what you have done.
And of course, you can do everything right and still not find a partner. But in a world that have sold you the idea that you *need* to have a partner and if you don't, then you are a loser and there is something wrong with you, of course the people who have "follow the plan/rules" feel that women aren't fulfilling their part of the deal. Even if women never asked them or wanted that they acted in a certain way.
The idea that to find a partner you need to do something more than "simply" get a job and that you need to work on real connection, specially if you haven't taught them how to connect with others, is like "changing the rules" and therefore "unfair". That is why they resent so much the feminist, because in their heads, feminist have changed the rules between men and women and "before, things were simple". If you bother to listen to grandparents, things never were simple, but some people think they were.
And of course, the changing roles, the uncertainty of the future and women who now ask for more than a wage from their partners doesn't help. And all the "alphanagging" and the "Chads" talk is an attempt to find a simple explanation for something that never were simple. Blaming the Chads and women and saying that what you need is to be an "alpha" is way easier that finding real connection to someone and that also, there is mutual attraction, and that all that is coupled with the other person being available and not in a monogamous relationship.
Probably Incel would benefit from other men telling them about the obstacles they have faced finding girlfriends and taught how to empathize and connect (which might not work, but gives you better chances at meeting someone).
Deal with frustration is always difficult and entitlement make it worse. The more you are supposed to succeed, the more you feel like a failure for not achieving your goals. That is the problem at its core. They need to learn that "losing" is just a step in the path, that you can learn from that and try again. And even if you don't success, that is not a problem with you, that it might be just bad luck or a system that was broken from the beginning.
Anyway, is just my take based on the premises they have. Where did they heard that "they deserve sex"? Where did they get the idea that "women owe them"? It turns out is all around the place. Movies, books, songs, publicity... Taken out of context: "Be the hero, get the girl" turns to "I did everything right and girls keep ignoring me. They owe me (for doing the right thing) and now aren't doing their part". that is why they insist on "men going to war" or "working" or "being the ones who discover/invented thing", even if they haven't done that personally and some women have done all those things. Which make it worse in their heads, because if women go to war, work and invent/discover things, then how are they supposed to get girls?
And to make it worse, some people are profiting from manipulating Incel. All those Pick up artist courses, or lessons on how to get women by being an alpha, blaming feminist and political discourse are like the make up commercials, they make money out of people feeling down/not good enough.
So I would suggest that reducing societal pressure to find a partner/have sex while celebrating different ways of being successful than "making lots of money/being a provider/having lots of sex" might help.
My guess is that many Incels have good things going for them and good qualities that they don't feel confident showing because they don't fit the alpha role. If they only show the bitter side, it is unlikely their situation improves, but maybe if they become confident on their other qualities (and are willing to look for more qualities on others) they would have better chances. Also, being nice to others usually help to being liked back. So I would say that they are following the wrong advice.
But as I said, it is just a theory.