On the face of it, it seems obviously wrong: people often do require effort and struggle to change, and evolutionarily speaking that seems like what one should expect.

Yes... and no. See below:

(You don't want random other people to be able to change your behaviour too easily, and easy self-modification is liable to make for too-easy modification by others.)

Exactly. The conscious mind is both an offensive weapon (for persuading others) and a defense against persuasion. Separating conscious/social ("far") beliefs from action-driving ("near") beliefs allows the individual to get along with the group while remaining unconvinced enough to continue acting impulsively for their own benefit under low-supervision circumstances.

In other words, willpower works better when you're being watched... which is exactly what we'd expect.

The offense/defense machinery evolved with or after language; initially language probably worked directly on the "near" system, which led to the possibility of exploitation via persuasion... and an ensuing arms race of intelligence driven by the need for improved persuasive ability and improved skepticism, balanced by the benefit of remaining able to be truly convinced of things for which sufficient sensory ("near") evidence is available.

You remind us frequently about what miraculous techniques you have. So it seems like by now you should be a walking miracle, a paragon of well-adjusted Winning. And yet, it doesn't seem all that rare for you to post something saying "I just discovered another idiotic bug in my mental functioning. So I bypassed the gibson using my self-transcending transcendence-transmogrification method, and I'm better now." To my cynical eye, there seems to be some tension here.

Only if you don't get that belief systems aren't always global. Some beliefs are more global than others.

Also, it's important to bear in mind that knowing how to change something, knowing what to change, and knowing what to change it to, are all different skills. I've known methods for the first for quite some time now, and the last year or two I've focused more on the second. This year, I've finally started making some serious progress on the third one as well, which is actually part of understanding the second. (I.e., if you know where you're going, it's easier to know what's not there yet.)

For example, Dweck's work on fixed vs. growth mindsets: that stuff isn't a matter of global beliefs in a literal sense. Each area of your life that you perceive as "fixed" may be a distinct belief on the emotional level, so each one needs to be changed as it's encountered. In the month or so since I read her book, I've identified over half a dozen such mindsets: intelligence, time, task granularity, correctness, etc... each of which was a distinct "belief" at the emotional level regarding its "fixed"-ness.

Changing each one was "magical" in the sense that it opened up a range of choices that wasn't available to me before... but I couldn't simply read her book and decide, "woohoo, I will change all my fixed mindsets to growth ones". The brain does not have a "view source" button; you cannot simply "list" all your beliefs on the basis of an abstract pattern like fixedness vs. growthness, or ones that involve supernatural thinking, or any other non-sensory abstractions. (Abstractions are in the "far" system, not the "near" one.)

OK, so there are some ways, commonly harmful but maybe sometimes exploitable for good, in which our mental states can be messed with non-rationally for a shortish period. Remind me, please, how that is supposed to be good evidence that we can consistently change our behaviours, motivations, etc., in ways we actually want to, with lasting effect?

We are constantly self-priming. Techniques that work, work because they change the data we prime ourselves with.

When you discovered that Santa Claus didn't exist, did you try to stay up late to see him any more, or did your behavior change immediately, with lasting effect?

Basically, you stopped priming yourself with the thoughts that generated those behaviors, because your brain was no longer predicting certain events to occur. It is our never-ending stream of automatically-generated internal predictions that is the main internal source of priming. Change that prediction stream, and you change the behavior.

External methods of change work by forcing new predictions; internal methods (including CBT, NLP, hypnosis, etc.) work by manipulating the internal representations that are used to generate the predictions.