I think the indirect method as proposed seems logical, but in practice would probably end up suffering from the same accusations ("unelected bureaucrats") that the EU commission suffers from.
So probably you need a directly elected chamber, elected on a PR basis probably by multination constituencies; which maybe in turn could elect a collective executive.
I also think a world government would require effectively abolishing the nation-state as a concept, and giving a strong level of autonomy to a network of sub-national regions and /or city states, to better represent the sorts of local concerns that a world government (and even nation-states) aren't able to deal with.
fwiw the cube root method gives a current size of just under 2000 members, with 2154 for a potential peak 10 billion population.
Quite simply, your hypothetical world parliament wouldn't need to be so big: there would still exist many legislatures below it for country specific things; and a parliament that size would be logistically unfeasible.
I dunno, the House of Lords is probably going to reach that size at some point.
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