Regarding Pine Island Glacier, it is it's volume that will determine when it will disappear and how much it will contribute to sea level rise. Volume is made up of 3 components - height, width and length. Pine Island Glacier is not only losing height but is also losing length. A similar scenario is occurring at Thwaites Glacier, with the second study mentioned is this article, and available at http://www.sciencemag.org/content/344/6185/735 concluding that early-stage collapse of Thwaites Glacier has begun. Citing the study's author, this NatGeo article says "Joughin says the breakup of the Thwaites glacier will resemble mechanical failure more than straight-up melting. The ice will slide into the ocean, where it will break off and float away, adding to the volume of water in the sea"
The Shepherd et. al. 2012 article that you quoted is at http://www.sciencemag.org/content/338/6111/1183 and actually has a fair bit of certainty about the ice mass changes in Antarctica. They say that they achieved greater certainty by combining satellite data sets, and that between 1992 and 2011, the ice sheets of Greenland, East Antarctica, West Antarctica, and the Antarctic Peninsula changed in mass by –142 ± 49, +14 ± 43, –65 ± 26, and –20 ± 14 gigatonnes/year, respectively; and that since 1992, the polar ice sheets have contributed, on average, 0.59 ± 0.20 millimeter/year to the rate of global sea-level rise.
A separate study, published in Geophysical research letters just after the two studies described in this NatGeo article, and available at http://onlinelibrary.wiley.com/doi/10.1002/2014GL060111/abstract analyses Cryosat-2 data and concludes that "Between 2010 and 2013, West Antarctica, East Antarctica, and the Antarctic Peninsula changed in mass by −134 ± 27, −3 ± 36, and −23 ± 18 Gt/year, respectively. In West Antarctica, signals of imbalance are present in areas that were poorly surveyed by past missions, contributing additional losses that bring altimeter observations closer to estimates based on other geodetic techniques. However, the average rate of ice thinning in West Antarctica has also continued to rise, and mass losses from this sector are now 31% greater than over the period 2005–2010."
The thing that matters is where those mass changes have come from and are going to. If the mass changes are caused by changes in the amount of/rate of snowfall we have less to worry about, but if the mass changes are caused by faster melting and faster sliding into the sea then we should be concerned about the effect on sea level.
The reduced effect of ice that is grounded below sea level due to it being "partially floating", and the fact that you have to melt a LOT of grounded ice to significantly affect sea-level have both already been incorporated in the calculations of sea level rise.
The anthropogenic change in CO2 levels would be better described as an elevation than a pulse. Despite some of the CO2 we produce being sequestered by the biosphere and the oceans (causing an increase in ocean acidity (or a decrease in ocean alkalinity, if you like) and a resultant drastic effect on marine life and potentially the world's food supply), the rest is predicted to remain in the atmosphere for a very long time. In comparison to the time scales involved in ice sheet changes, this article and many others actually show that the previous conception of multi-millennium time scales needs to be revised based on evidence of changes occurring across much more rapid time-scales.