Are all types of hydronephrosis treatable and curable with surgery? What about congenital hdn without a stricture or reflux?

Some types. There are few causes of nonobstructive hydronephrosis. Infection and pregnancy are some of them. Treating the infection may reverse the hydronephrosis. Eventual delivery can reverse the physiologic hydronephrosis of pregnancy. A urologist or nephrologist may have more examples for you.
No, if the hydronephrosis is long standing and there is permanent damage to the the kidney, there may be irreversible partial or complete loss of renal function. At that point, the system is dilated but no longer under pressure.