Hello Cathryn.
1: Yes, if an island was completely washed over, the maximum elevation of the island would be less than the run-up height. But, I think in this area there are plenty of big islands that were not completely washed over, so there will be a very good idea of the height of the islands.
2: Satellites could have imaged the tsunami damage...and I even saw a before and after of a satellite image on TV news. But, the high resolution satellites are probably not set up to do this quickly enough: only geostationary satellites, which are relatively far from the earth, sit over one spot on the earth. The others move relative to the surface of the earth, and only pass over a particular location once in a while...every couple hours to every few days or weeks, depending on the satellite. But, even if a statellite was fixed on northern Sumatra, it would have to be set up so that the images were being made available to analysts in "real time" (immediately)....and probably only in Iraq, and only with military/classified satellites, would images be analysed very quickly.

3:GPS could have worked. Let me answer that in a new thread, as this is pretty far down the page.
Chris