E.g. what initially set events in motion in Berkeley seems to have been mostly MIRI-related research, not explicit community building.

There are many different reasons for a computer programmer who's interested in rationality to move to Berkeley besides MIRI-related research. The good jobs for computer programmers are in Silicon Valley and so computer programmers migrate there.

And it seems very possible to choose this kind of focus top-down.

If you have enough money than you can do everything top-down. If I understand right than MIRI had over a longer time >500,000/per year in donations. Do you think there somebody who would want to invest that kind of money into building a European LW community?