> (I’m not sure why the data from my old domain, cyhsu.xyz, hasn’t been aggregated to the new hsu.cy, despite the methodology page saying it should. Must I return the canonical header in addition to a 301 code?)

It's a manual process, so I have to do it by hand when I notice a domain has moved. I've just added yours and kicked off a reprocessing job so that your old domain counts toward hsu.cy.