From the file, I think you want the sum to just ignore the "+" sign.
You now have
coalesce(${megalithal_stufen}, '0') + coalesce(${makrolithal_stufen} , '0') + ...
I think it could be if the variable is "+" then use '0', else use coalesce(...):
if(${megalithal_stufen} = "+", '0', coalesce(${megalithal_stufen}, '0')) + if(${makrolithal_stufen} = "+", '0', coalesce(${makrolithal_stufen} , '0')) + ...
(check if() documentation)
Also, I think E17 and E18 can be empty (required column), I think you only need L17 and L18 (relevant)