Komplizierte Abfrage geling nicht.

Das kann man schon vereinfachen, indem man die berechneten Felder in eine sog. Derived Table packt:

select x."T_Messungen_ID",
       x.id1,
       x.id2,
       x.px1,
       x.py1,
       x.px2,
       x.py2,
       py2 - - py1 / px2 - px1 as m_steigung,
       ... und so weiter ...
from (
  select t1."T_Messungen_ID", 
         t1."T_Messungen_Daten_ID" as id1,
         LEAD(t1."T_Messungen_Daten_ID",1) OVER w as id2,
         t1."T_Messungen_Daten_x" as px1,
         t1."T_Messungen_Daten_y" as py1,
         LEAD(t1."T_Messungen_Daten_x",1) OVER w as px2,
         LEAD(t1."T_Messungen_Daten_y",1) OVER w as py2,
  from "T_Messungen_Daten" as t1            
  where t1."T_Messungen_Daten_Art" = 5 
    and t1."T_Messungen_ID" = 125 
  order by t1."T_Messungen_Daten_x";         
  window w as (order by ORDER BY t1."T_Messungen_Daten_x")
) x