[IXRollOutIX]

What? Where was it ever said the soul is inside the host? Helios is more accurate, but I'm not sure the word "sealed" is exactly right, (although the eye symbol Ghost draws is referred to as a seal of some kind, I believe.)
Are you sure you've been paying attention, Suzu? Because it's pretty clear the souls are dormant within an item which was close to them when they lived. To awaken their spirit, it requires any human who resonates with their goals and beliefs (or something like that.) Embracing the positive ambitions of the soul results in sparing the host. Reflecting their negative traits resonates adversely and the host is sacrificed. The biggest clue that the soul is directly related to the object, and not the host, is that when Takeru draws the eye symbol, the item is consumed and becomes the Eyecon itself.

As for the Houdini situation, I think it's a special case. Houdini's soul is probably still in the Eyecon and activating it in the Driver calls them like normal. It's just that instead of becoming a parka itself, the soul manifests into the bike and turns THAT into its "phyiscal" form.