You've pretty much got it. I would add
1a) The proposed existence of Nibiru makes certain predictions including a regular disruption in the orbits of the planets, including earth, which should show up in the geological record. No such cycle is observed.
1b) If Nibiru existed, and had made a close pass to earth in the past, then we would not expect to see earth in a stable orbit with a low eccentricity. Since earth is in this orbit, that is evidence against the existence of Nibiru.
1c) Nibiru advocates propose a global conspiracy involving all of the world governments, every astronomer, and every hobby stargazer with a telescope larger than 6 inches in order to 'hide' the existence of Nibiru from the world. On its face, this argument fails.
1d) Nibiru ( as predicted by Zacharia Sitchin ) is not supposed to approach earth until 2085. The 2012 date is a co-option of the "Nibiru" myth into the "Mayan Calendar" myth.
1e) A body that large would stabilize its orbit, and would not be found on a 3600 year highly elliptical orbit taking it from outside the orbit of Neptune to a distance of 1 AU from the Sun. It should have stabilized into a much more circular orbit by now.
1f) There is no evidence from any perturbations of the orbits of Uranus and Neptune that there is a large undiscovered body out beyond Neptune. Claims to the contrary are using data that is decades out of date. These data have been refined using observations from the Pioneer and Voyager spacecraft. There are no known 'anomalies' in the orbits of the outer planets.
2a) the Mayans made references to events that were predicted to occur after 2012.
3a) There is no 'alignment'. The line running between the centers of mass of the Earth and Sun does not intersect the center of the galaxy. It is off by almost 6 degrees.
3b) The sun will not occlude the center of the galaxy. The angular diameter of the Sun is slightly more than 1/2 a degree.
4a) The predictions of a larger than normal solar cycle in 2012 are made in the face of great uncertainty of the accuracy of predicting solar cycles. The consensus among astronomers is that we currently can not predict the peak strength of solar cycles.
4b) The above prediction made by a solar scientist indicated that the prediction was uncertain (i.e., speculative ), and also predicted that it *may* reach levels of the 1956 solar maximum.
6b) The field strength of earth does not drop to zero during a magnetic polar shift.
6c) There is no consensus among geologists that a magnetic polar shift is underway. In fact it appears to be quite the opposite.
6d) The magnetic polar fluctuations generally take hundreds or thousands of years to complete.
6e) The shifting of the magnetic poles is not run by a clockwork mechanism. Pole shifts have occurred regularly during some epochs, but have remained in one orientation for as long as 40 million years during others.
7) is not quite accurate. There is an approximate 11-year cycle of sunspot activity. This is magnetic in origin, and not tied to the temperature of the sun.
8) IRAS data (often cited by Nibiru advocates) from 1982 or 1983 did see an infrared object with no corresponding visual object. However, on subsequent observation this turned out to be a new (at that time) type of galaxy that was only visible in Infrared. IRAS did not see a 'planet' as some claim.
9) The claim that Nibiru exists is the positive claim. It is up to the advocates of that position to provide the proof that this object exists.
I've got some links you may want to look at at the end of my 'debunking' article (link below). Just scroll down to the bottom and read the links that go to "Universe Today" and "PseudoAstronomy"
EDIT: Corrected 3a... it's 'almost', not 'more than', added 9
