As you can see in question, a lot of things were checked and a lot of debugging was performed, but the most elemental thing was not checked. The user in question had simply visual editor disabled in user configuration:
After enabling this issue become solved.
This is dumb and underlines my ignorance. The only explanation I have is that this particular user was created eight years ago and so was its options set.
I decided to keep this question open, so it can benefit others that may run into similar troubles in the future.
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