The 1% number is a percent of impacts not deaths if I interpret this article
and other sources correctly. It was been previously stated in those other
reports that comets represent about 25% of the potential risk in terms of
fatalities. It was also stated that little could be done to prevent them
due to a combination of speed, size, and short notice. This residual risk
is not within the realm of near term technology to deal with. We can't get
more than about 6 months warning for long period comets and we couldn't
divert them even if we got that warning. We need far more powerful space
launch technology and/or an additional home for humanity to eliminate
impacts as a threat to human civilization or even downright extinction. I
believe this change in terminology focus is part of what Morrison was
referring to when he mentioned seeming contradictions with older reports.

Space - its about survival not just curiosity. This is part of why the
space program should focus on colonization potential (where Mars wins hands
down over any other site in the solar system) not just scientific potential
(where Mars is also near the top of the list, but not in a class of its
own).

With regard to your questions on risk I think they are referring to reducing
short term risk by eliminating or identifying the portion of long term
average risk that we would expect over the next few centuries. There is a
reasonable probability that we do not face the statistical average risk over
that period because the risk is episodic rather than continuous with impact
intervals frequently longer than that period. If we remove most NEO's from
the list of potential impactors we will know the near term risk is lower
than the long term average. If we identify potential impactors we will give
ourselves a chance to divert them which will also reduce the risk. There is
a good chance there will be no real reduction in risk if no intervention is
needed, but rather a reduction in the known or estimated risk.

Regards,
Mark