Hi Stefan,
Usually what Google demands (and "normally" people would / should do) is to model with Terrain on since if it is 3D,the terrain should also be 3D.
However,with simpler models (one building at a time) when you suddenly turn terrain off in GE, the model should more or less "accurately" jump down or snap to the flat terrain.
This cannot be achieved with a longer wall of even the smallest graveyard (especially knowing that these graveyards are actually often placed on hillsides or hilltops near the villages in the valleys in Europe).
A solution could be to model smaller pieces of the wall separately and upload them one by one but this would be a ridiculous demand so I would go to ignore those not toggling terrain on when viewing 3D buildings.
Of course there could be a solution; to prevent 3D building from being displayed when terrain is off in GE - or more exactly, turning on the 3D Layer would automatically trigger the terrain (and with exaggeration 1).
Would make a good feature request