Tobias,
– I think in its category/essence, moral or immoral, it is kusala; punna is inherent in kusala and is the fruit or manifestation of it. So I would say that kusala becomes punna, rather than the other way round.
– Yes, your understanding of English is correct; for the sake of grammatical accuracy it should be ‘next lower strenght’, but I doubt whether anybody misunderstood what Lal meant. In such clear cases, not catching the error is what prevents the misunderstanding.