Twice a year, but yes. That's a huge benefit -- not only is it easier for patients to stay on the treatment, but it's likely to be a lot cheaper as well.
Probably quite a bit. The trial used the same dose of lenacapavir as what's used for maintenance in HIV patients; it's quite possible that less is needed to prevent infection in a healthy patient. Unfortunately, there's really no safe/ethical way for them to test lower doses.