Likely, I agree.
That said, the (Old) Heterodynes frequently raided the surrounding lands, and sometimes spent years away at a time. I’d really think one would die in foreign lands at some point. Given how powerful Castle Heterodyne was, I’d also think someone would have tried the cloning by now (especially since they clearly don’t need the entire body to grow a clone, or even most of it).
There are other explanations—maybe cloning is a relatively new technology (Though then why the long-establishes 50 families rules on resurrections? They seem to be similar procedures...)—but my gut says some of those skulls in the chapel were clones. I wonder if Castle Heterodyne can feel the spark somehow?