The text is a bit difficult to read. Better formatting would probably help.

I wonder if there are two strategies, each with some advantages and disadvantages. With "open door" strategy, you talk to strangers. You may convert them on your side, but also they may convert you or your friend to their side. With "closed door" strategy, you cut communication. No gains, no losses.

It seems that currently the fringe left prefers the "closed door" strategy, while the fringe right prefers the "open door" strategy. And the right seems successful, like making Trump president, or the recruitment by niceness you mention. However...

First, I doubt whether this is a good description of current state. USA is a de-facto two-party state. It is perfectly normal that the parties alternate at power. Democrats had their turn, then Republicans had their turn, now Democrats have their turn again. Business as usual. (What would you expect instead? Democrats winning all the time?) Arguably, the actual victims of Trump are the "mainstream Republicans"; they didn't have a president for the last 12 years, and are not going to get one for another 4 or more likely 8. That's an entire generation!

Reporting on alt-right is exaggerated. By "homogeneity of the outgroup", all kinds of politically incorrect groups are called alt-right, even those where 90% of people identify as left-wing. So "alt-right uses niceness to recruit people" probably only means "some people are nice to each other on internet... and then they are denounced as alt-right". That doesn't sound scary for the same reasons.

Second, assuming that we accept the premise, the right obviously has no incentive to change their behavior. Does the left have such incentive, though? Consider the fact that higher education is traditionally the bastion of the political left. Students grow up indoctrinated with left-wing beliefs (which they usually don't even call "left-wing", but simply "common sense" or "science", because that's what everyone around them believes), and as they become adult, they gradually shift to the right. (Someone probably made a proverb about it centuries ago.) So who is going to benefit from the "closed doors"? Seems to me that the left -- it leaves their recruiting centers intact, but slows down the falloff.

Sorry for the political rant. I don't actually care about the alt-right. Rather, I am worried about the meme: "So you say people should be nice to each other? You know who else was nice to people? Hitler!" Because, I like it when people are nice to each other. Yes, I realize this is exactly what a Nazi trying to infiltrate an online group would say.

You didn't talk about absurdity at all in your proof about absurdity.

In my opinion:

"The closer an idea is to what you already believe the easier it is to think of it." -- Yes.

"The closer an idea is to the truth the easier it is to think of it." -- No.

These is this idea of systematic bias; of errors that all people do for the same reasons (e.g. because making this type of error often provided an evolutionary advantage, or because the neural networks are likely to make this type of error) Ideas like "there are supernatural agents that act in our world" are easy, discovering electricity is hard.