Is my 12v lead acid battery 100% dead?
During over discharge, sulphation occurred on the plates.  If porous, this sulphate can be reversed by charging.  

From Storage Battery Troubles under Sulphation - Over discharge


  If the discharge is continued after the voltage has fallen to the voltage limits, an excessive amount of sulphate forms. It fills up the pores in the active materials, and covers up much of the active material which remains, so that it is difficult to change the sulphate back to active material. Moreover, the expansion of active material which takes place as the sulphate forms is then so great that it causes the active material to break off from the plate and drop to the bottom of the jar.


You are able to partially charge the battery, so you have changed the areas of fine sulphation back into plates of lead peroxide and spongy lead, but the integrity of the plates are damaged.  The grid structure is fragile if the battery is in an uncharged state.

The cells are probably damaged with loose material.  It cannot be repaired.