Wonder when they'll put up a female Ganma to join in? Since Aran, his brother and Javelle.

If the Houdini Parka Ghost is inside the Machine Hoodie, then what's the purpose of the eyecon? If the eyecon doesn't contain the Parka Ghost, then can't he access the form without the eyecon?

The Eyecons are linked to the Ghosts. And Makoto already had the Houdini Eyecon from the very beginning so I'm not really sure what you are getting at with your question.

The Parka Ghosts come from a sealed item. After the item is unsealed, they enter the Ghost Driver and create the Ghost Eyecon which houses them. If Houdini is inside the bike then is the eyecon empty? I guess the eyecon can activate the finisher, but it doesn't contain the Parka Ghost.

The Ghosts are not sealed inside items- where did you get that from? The Parka Ghost sleep inside their host and when they are released, they enter the Eyecons.
The items that were connected to the hosts were mostly stuff used by the Ganma trying to lure the Ghost out by killing the host.

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.

The current episode, for example? Ryoma was clearly residing inside the hosts body and not an item.

But only AFTER he had already become an Eyecon. Did you miss when the host found Ryoma's completed Eyecon in a bush and it possessed him just as a standard Ganma Eyecon could? The events that happened in these last episodes have nothing to do with the Eyecon summoning process, and more to do with unrelated possession after the fact.

Well, my mistake then. Nonetheless, the spirits are not sealed inside items- or at least not necessarily. My best example is sadly one most people haven't seen yet. In Movie Taisen Genesis, Da Vinci is not sealed inside his works, he does need the host to be freed, which is why the Ganma to use his eyecon kidnaps the host (saying who it is would be a massive spoiler, but that's what happens).

And it was never said that Houdini is sealed in the Bike. As far as we know Hiudini IS the Bike. Makoto just cant use its Parka Mode.