But then dropped the whole thing
they figured out the Cylon disease
Another good point, how exactly did they figure out that this virus would give Cylons a disease and yet at the same time they still don't have a Cylon detector that would've picked up Tigh and the others? The former would require some actual biological knowledge of Cylons and in particular how Cylons differ from humans. Which would imply the ability to distinguish Cylons from humans. Which, in turn, would have meant they had the makings of a Cylon test right in front of them. And yet they never put this 2 and 2 together.
Cottle figured out that it was a disease that humans used to suffer from but had developed an immunity to millions of years ago. They’ve given up on Cylon detectors after Baltar’s “failed” (remember it worked, he just lied about the results). By the time they found the Cylon disease they knew what the 7 known models of Cylons were, and since even the Cylons didn’t know what the Final Five were like there really wasn’t much reason to work on a test to find them. I’m sure once they got off NC all the testing on Cylons was geared towards immediate thing like figuring out the disease, since they have almost no doctors (you can cook up all the ways they could train up “doctors” you want they still have almost no real actual fully trained Doctors, and the ones they have are working 16 hour days just keeping the fleet alive) and they already know what the 7 that were part of the genocide look like, there’s really know compelling reason to look for general differences. There’s no reason for them to look into some Cylon legend of “missing” Cylons that might or might not exist, there’s people with diseases that need cures.