i think that if xavier's got enough knowledge to map the genetics of all the mutants in his teams of x-men, as well as the affiliate teams such as x-force and excalibur, he's got enough knowledge to know whether or not decapitation would kill 'im. and you comparing xavier's knowledge of a mutant very close to him to a scientist's knowledge of "all sciences" is not only a poor analogy, it's poor debate tactics.
“Since the loss of his adamantium, Wolverine has become both more and less of a threat. He is divested of his unbreakable skeleton - but his senses and his mutant healing factor have been increased to incredible levels. What would have been a lethal wound to Wolverine three years ago would now be a momentary annoyance. Any assault on Wolverine, in the event of his sanity breaking, would have to be long-range and decisive. His head would have to be severed, and removed utterly from the vicinity of his body to prevent swift flesh and nerve regrowth.“
this implies that the only way for wolverine to recover would be for the last electrical impulses in his body to stimulate his cells' ability to grow back together. a poor way to decapitate him would be through the use of a knife, or even a piano wire, which would require prolonged sawing to get through his neck. however, once his head is completely severed, you need only turn it so that the two wounds don't face eachother, and thus can't meld back together. you needn't punt it a quarter-mile away so that his headless body can't retrieve it; it simply can't grow back together without the injured areas of the severed neck being mere centimeters from eachother.
and, also, that explanation makes the possible "decapitations" in issue 32 IMPOSSIBLE. they would've killed him instantly, considering he was operating on that less-powerful healing factor. so, sorry. he still isn't immortal, and he still isn't coming back from the dead.