That's...actually a really good idea :o All of Seth's previous creations he let loose, until Pale's notoriety made him decide to stop making clones of himself. Kiril and Pale were his first creations. So he couldn't have made Keel (or he changed his mind for completely arbitrary reasons even after improving the process so he wouldn't have to clone himself).
Irina could have worked under Seth's tutelage (possessing a Rin medium, probably) after he revived her. He could have survived long enough to pass on some of his knowledge and DNA for her, then told her to continue experimenting in his stead. Thus, she made the Freezis children, either under orders to create a new body for him that could be used when the time is right, or just missing her mentor. This would explain what she was doing between getting her cat body and seeing the 7 lights, as she still apparently hadn't considered getting revenge on Elluka until then.
It's also possible she started the experiments after Chrono Story--it doesn't affect things much if the order is different.
This might be why Keel isn't evil (although he could just be an HER who managed to fight off his condition). She might not have perfected the process of implanting the virus in the clones' genes.