I know nothing of this field, is there any idea of how long "information" stays in the brain or any metric like that once blood/oxygen flow to the brain has been inhibited?
E.g. people found drowned in icy water with no pulse. "Six of nine survivors (66.7%) had minor neurological sequelae with Glasgow outcome scale (GOS) 5 (low disability)" (the rest worse). It's not clear to me from the table what the longest time with no breathing or pulse was among those six best-off survivors. But here's a popular account of another case: http://www.iflscience.com/health-and-medicine/child-survived...
Those were the top two google hits that came up. It's not my field, but it doesn't seem plausible to me that much of our identity is encoded in evanescent patterns of electrical activity or such. (As opposed to your short term working memory.) If you can nail down the molecules, the information is bound to be there in their arrangement.
Not that I've tried, but I guess detaching a head at the base has less potential to damage than would opening and removing the whole skull.