The most likely hypothesis seems to be that there is a lot of interstellar material around the star, such that the ejecta is colliding with that and keeping it hot to keep glowing. It was a core-collapse supernova initially; the energy released from that is being absorbed and more slowly re-emitted by the thick surrounding material. (Thick in astronomical terms is still something like a microgram of hydrogen per cubic meter.)

Yeah, that's roughly what I figured (IANAA). But it still seems hard to explain six flashes bright enough to appear like a supernova. I'd expect the flash of any kind of supernova to be brighter than the flash of a shockwave hitting a shell of gas. But I know nothing.

And anyway, for the shockwave from a supernova explosion to hit the shell of expanding gas from a previous explosion, that still seems to imply two supernova explosions from the same star. The only way I know that can happen is in a binary system.

Perhaps it’s a hybrid? Original core collapse, then accretion of in falling material after the shock from the last explosion dissipates, causing spontaneous fusion/degeneracy again and re-explosion?