You discuss institutions, but I don’t think you discuss the right kind of institutions. If I am comparing Malawi to some of the other nearby landlocked African countries you mention, the first thing that jumps out to me is their dramatically worse economic freedom. Malawi has one of the worst scores in the entire world, ranked 147⁄165, a level more typical of central or saharan african countries.
(You didn’t mention Eswatini or Burundi, but they also score very badly—unfortunately the map tool above will only let me display 5 countries so I focused on those named in the text plus Zambia as it is neighbouring).
This doesn’t resolve the infinite-regress style question of what causes some countries to have more capitalist institutions than others, but when it comes to which institutions to investigate, I think it is their economic institutions we should focus on.
If you weren’t using a login, presumably you were using the lowest tier of models, which I don’t think is a very good test.