It is anxiety on her part, and the therapy for anxiety is gradual desensitization, also called Exposure therapy. CBT - cognitive behavior therapy - has also shown a decent amount of success for anxiety. See Therapy for Anxiety Disorders: Cognitive and Behavioral Approaches

I do not see how marrying her would help alleviate the anxiety. If the idea is that marriage is the ultimate guarantee that you - unlike that other guy - would not leave her, then, yes, I can see the point, but that point would only be helpful for that subcomponent of her anxiety which is the fear that having sex leads to being abandoned. Her anxiety has many more subcomponents which are not going to "yield" to marriage. Say, you wrote "she has (since the scarring Japanese sex ed videos of her childhood) come to fear the organ between her legs. " - how is marriage going to help with that? I think the idea that marriage will fix the problem is an example of magical thinking.

If you know enough Japanese, make some searches for therapists who treat anxiety in one of those modalities - Exposure and CBT - and offer her some choices, making sure that she knows that you do NOT have quantifiable expectations of her progress. In other words, find some professionals and express a hope that maybe, with therapy, one day and with absolutely no deadlines, your gf would be comfortable enough to proceed to intercourse.

So that is about your gf, now about you - why do you feel insecure just because you have not had PIV (penis-in-vagina)? You have done, per your report, a variety of things sexually; you report being sufficiently and even more than sufficiently equipped in this department (you worried that she was too small for your penis) - so what is that makes you feel insecure? You wrote about this insecurity as if it were obvious that one in your situation would be insecure - at least for me, it is not obvious, so I am asking the question.