We visited the Kilauea Wildlife Refuge in March of this year. We were able to gain admission by showing a copy of our tickets to the park ranger (i.e., we didn't need to provide a QR code). Unless they've since changed something, you should be able to provide a copy of the ticket or similar proof of purchase to gain entry. Please note that the Wildlife Refuge doesn't have good reception/signal for phones and whatnot. Make sure to either have a physical printout of the tickets/email or a copy readily available on your phone prior to arrival. Some visitors had issues because they couldn't pull anything up on their phones at the entry gate.

From a local Kauai newspaper: "Kilauea Point National Wildlife Refuge is closed due to the government shutdown. On the refuge’s website, a notice advises: “Where public access to refuge lands does not require the presence of a federal employee or contractor, activities on refuge lands will be allowed to continue on the same terms as before the appropriations lapse. Any entry onto Refuge System property during this period of federal government shutdown is at the visitor’s sole risk.”" This was from Dec 28 though. Many National Parks and Monuments have now completely closed due to problems with visitors leaving trash and overwhelming the capacity of the restrooms. I would assume that it is officially closed even to walk-ins at this point.

10 am to 4 pm. Closed only on major holidays.

You can definitely buy one here.

Sure does. I actually bought my pass there when I turned 61.

Yes. Senior pass accepted.