What is considered a decisive advantage be the computer?
In other words what computer evaluation would guarantee a win if perfectly played?
In can be a undefendable +0.1 where as a fortress can have +9 (draw).
So what do you mean? A position can only be won or it is a draw.
AFAIK the computer evaluation is a "measurement of confidence" rather than "this is a win" unless the evaluation is "mate in X". So +5 means the computer thinks there is a win for white but it can't find one (at that depth). I've seen GM resignations where the final position has been 2.5 or 3 (for white or black), but those positions would be difficult if not impossible for someone like me to convert to a win.
+1 is usually won, but there are exceptions as per #2
+1.5 is 'technically won', +3 'easily won' in 95 % of cases. I dont think +1 is enough, thats still draw against perfect defense in most cases.
@BlackSalt that post is a joke. Unfortunately it was made 3 days after April Fools which means people are taking it seriously.
The article of #6 was published on 4/2/2012, one day after 4/1/2012 when the interview allegedly took place...
No matter how deep a machine calculates - after the horizon everything can happen.
How long has the straw in your hand to be that you always win?