Missing Passthrough Scenario

I have not made any progress.

I understand your confusion; it appears that a lot of the documentation was written with the Quest Pro in mind and was written before Quest 3, and things have straight up just changed without the standard deprecation period that most of us are typically expect.  The closest thing to "pokeahole" is "User Defined Passthrough in Overlay mode".

But as I mentioned above, I was not happy with this solution because it does a reprojection of what the camera sees onto this geometry, and the geometry slightly warps things to the point that moving the HMD results in vomit inducing shifting of the world.  I believe the Reconstructed Passthrough looks so good because it is using the room geometry and doing the camera projecting onto that geometry, which gives us the quality that we've come to expect and love.  But there does not appear to be a way to "pokeahole" in one's seen so that we can see the Reconstructed Passthrough.

If you want to match the Quest 3 demo game, you can still achieve this by using Reconstructed Passthrough in underlay mode, and then drawing your world on top of it, which turns the problem into "How do I render a portal in Unreal with Vulcan forward rendering?"  for which I do not know how to do it.  I suspect the demo was written in Unity.

If my understanding of the problem space is correct, this means one won't be able to create a "portal" into your game's "virtual world" in the middle of the player's play space and have that player step inside, and then have it behave like they are inside it, because managing the transition from pass through with portal, to stepping half way through and having one eye see "from the other side of the portal" while the other eye is still outside of the portal, won't be possible, and will require a complex and non-visually-ideal solution.  You may not be attempting this route, but I was and that is why I gave up (Note: I got close to exactly what I wanted, but the shifting of the pass through with user defined geometry does NOT feel good unless that user defined geometry closely matches the location of the real world objects that are being projected onto them).