I have another bit of solid evidence to support Johnny being a Medium, which I believe cinches the argument. If you go to the Archives tab of the comic, there's alt text summarizing the page assigned to /each and every pages link/, and chapter 4, page 143, has hovertext that explicitly states "Johnny is possessed by Forge".
Also interesting to note, is that same page (as well as the previous page) is enough to disprove the old disqus theories that members of Johnnys' crew could be spectrals or mediums themselves, as all of them were able to pass through the barrier (which is a tad disappointing, as I really did think RJ was hiding a bit of spectral secrecy under that hood).
I do feel as though Max having been possessed shortly before entering Mayview is plausible- we've got canonical precedent with Johnny and Forge now, as well as Forges initial conversation with the Shadow Bunnies where they proposed having him possess an object as a means of bypassing the barrier ( http://www.paranatural.net/comic/chapter-4-page-46 ).
Using this context, we can postulate that a possessed human that has not yet awakened to the nature of a spectral counts among the same lines as a possessed object- and that exposure to a spirit will create a spectral in a short amount of time. Max began to awaken as a spectral the evening he arrived in Mayview, seeing shades of a ghost and hearing Hissing Pete- and he was still seeing shades well into the next day. Johnny is just now starting to see shades after being possessed by Forge.
I've gotta say I'm falling in line with the "Max was able to pass through the barrier because he wasn't exposed to spectral energy until just before he moved to town" theory in this thread- making BERG/Riddle an infiltrator similar to Forge, and apparently opposing (or at least not on the same side as) Doorman, and in relation, Forge.