1) Technical Installation as I have mentioned above, is the activity of linking the device to the device location.
2) I trust the easiest way would be to hit the EASTL table by passing the installation to the field EASTL-ANLAGE and EASTL-BIS NE 31/12/9999. This would give you all the devices which have been removed from the Installation. Use the EASTL-LOGIKNR and pass it to EGERH, and it will return you the equipment numbers ( EQUNR ). Now you can pass these EQUNR to the table EQUI which will return you the device serial number ( SERNR ).
3) Please do not confuse yourself with EGERR for now. Its a different concept. Hope point 2 clarifies your query in point 3.
Hope this helps.
Regards,
Mohammed Muzammil.