You’re correct. The HOMO is scf_wfn.nalpha()-1 and the LUMO is scf_wfn.nalpha() for a closed-shell system. For an open-shell system, determining what you mean by HOMO and LUMO is more complicated.
As this is a sufficiently long topic that the context of previous posts hurts rather than helps, I’m going to lock it. Let’s put further discussion in a new topic.