Bom tarde,
Após o detalhamento do seu caso, fiz vários testes, quebrei a cabeça e cheguei a uma solução.
Insira essa consulta no Editor de SQL, confirmando que está marcada a opção "Executar o comando SQL diretamente":
SELECT *,
"Jan Rend"-"Jan Desp" AS "Jan Saldo",
"Fev Rend"-"Fev Desp" AS "Fev Saldo",
"Mar Rend"-"Mar Desp" AS "Mar Saldo",
"Abr Rend"-"Abr Desp" AS "Abr Saldo"
FROM
( SELECT *,
COALESCE("Jan cartão",'0')+COALESCE("jan fixos",'0')+COALESCE("jan mercantil",'0')+COALESCE("jan outros",'0') AS "Jan Desp",
COALESCE("Fev cartao",'0')+COALESCE("fev fixos",'0')+COALESCE("fev mercantil",'0')+COALESCE("fev outros",'0') AS "Fev Desp",
COALESCE("mar cartao",'0')+COALESCE("mar fixos",'0')+COALESCE("mar mercantil",'0')+COALESCE("mar outros",'0') AS "Mar Desp",
COALESCE("abr cartao",'0')+COALESCE("abr fixos",'0')+COALESCE("abr mercantil",'0')+COALESCE("abr outros",'0') AS "Abr Desp",
COALESCE("jan salario",'0')+COALESCE("jan outras_rendas",'0') AS "Jan Rend",
COALESCE("fev salario",'0')+COALESCE("fev outras_rendas",'0') AS "Fev Rend",
COALESCE("mar salario",'0')+COALESCE("mar outras_rendas",'0') AS "Mar Rend",
COALESCE("abr salario",'0')+COALESCE("abr outras_rendas",'0') AS "Abr Rend"
FROM
( SELECT SUM( "valor" ) AS "Jan cartão" FROM "gastos_cartao" WHERE "mes_referef" = '0' ),
( SELECT SUM( "valor" ) AS "Fev cartao" FROM "gastos_cartao" WHERE "mes_referef" = '1' ),
( SELECT SUM( "valor" ) AS "mar cartao" FROM "gastos_cartao" WHERE "mes_referef" = '2' ),
( SELECT SUM( "valor" ) AS "abr cartao" FROM "gastos_cartao" WHERE "mes_referef" = '3' ),
( SELECT SUM( "valor" ) AS "mai cartao" FROM "gastos_cartao" WHERE "mes_referef" = '4' ),
( SELECT SUM( "valor" ) AS "jan fixos" FROM "inserir_gastos_fixo" WHERE MONTH( "data" ) = '01' ),
( SELECT SUM( "valor" ) AS "fev fixos" FROM "inserir_gastos_fixo" WHERE MONTH( "data" ) = '02' ),
( SELECT SUM( "valor" ) AS "mar fixos" FROM "inserir_gastos_fixo" WHERE MONTH( "data" ) = '03' ),
( SELECT SUM( "valor" ) AS "abr fixos" FROM "inserir_gastos_fixo" WHERE MONTH( "data" ) = '04' ),
( SELECT SUM( "valor" ) AS "jan mercantil" FROM "gasto_dinheiro" WHERE MONTH( "Data" ) = '01' AND "area" = '101' ),
( SELECT SUM( "valor" ) AS "fev mercantil" FROM "gasto_dinheiro" WHERE MONTH( "Data" ) = '02' AND "area" = '101' ),
( SELECT SUM( "valor" ) AS "mar mercantil" FROM "gasto_dinheiro" WHERE MONTH( "Data" ) = '03' AND "area" = '101' ),
( SELECT SUM( "valor" ) AS "abr mercantil" FROM "gasto_dinheiro" WHERE MONTH( "Data" ) = '04' AND "area" = '101' ),
( SELECT SUM( "valor" ) AS "jan outros" FROM "gasto_dinheiro" WHERE NOT "area" = '101' AND MONTH( "Data" ) = '01' ),
( SELECT SUM( "valor" ) AS "fev outros" FROM "gasto_dinheiro" WHERE NOT "area" = '101' AND MONTH( "Data" ) = '02' ),
( SELECT SUM( "valor" ) AS "mar outros" FROM "gasto_dinheiro" WHERE NOT "area" = '101' AND MONTH( "Data" ) = '03 ...
(more)