I agree with most of what you said, KWilliams.
I don't buy that Slimer was “made” in some ritual. He's got way too many human characteristics - and then the “how” comes into it. Like you said, there's never been any actual case where someone has claimed to have “created” a ghost from nothing. You say “except in Hollywood”, though, but I don't think Ghostbusters falls into this. Dan Aykroyd wrote the concept and the script and his strong interest in the paranormal means it's obvious he'd have based most of his concepts on facts and theories.
IF Slimer was a result of a ritual in the basement, he must have been summoned rather than “created”. So he would have already existed, but summoned to the hotel. You see this in the sequel when the Scolari Brothers are summoned by the emotions of the judge - it looked like they were created from the slime, but they already existed as they were once alive.
Anyway, about Slimer in the comic, this is definately him. I think the way he's been introduced is awesome. I was expecting a crappy introduction like “oh look, it's that first ghost we caught, let's get him!” type of thing. But in this he's not even mentioned as who he is - he's just there, waiting to attack Ray. It looks so sinister and is a great way to introduce him.