An interesting theory; it explains the original Trill from TNG very nicely. I'd make them benevolent after ancient history was over, though, to mesh with DS9. TNG's Trill can be the exception, or be a very ancient Trill.
I'm happy with that, too. Odan could simply have been an old and obnoxious specimen who took no pity on its hosts - never bothering to use their names as part of its own, even.
Possibly such old geezers are in control of the Trill society, though, explaining the weird dark secrets they seemingly unnecessarily protect.
How do we know that the Trill creature's minds don't infect and integrate themselves into the humanoid's minds in the same way that a computer virus or malware installs itself on a hard drive?
We could ask a host who has been liberated of his or her Trill. But Jadzia (in "Invasive Procedures") was only liberated of Dax for a brief moment, and was still suffering from post-surgery confusion, so we can't count on her testimony. Yet Riker was possessed for a while, then released, and reported few aftereffects... And if Odan really was a rude example, wouldn't it have done its very worst on Riker? Or was it so cunning as to realize that Riker would have to be released soon again, and thus held back with the subjugation viruses?