The two diodes are redundant. In normal operation, they only cause an additional small voltage drop. A series connection can increase the peak inverse voltage. However, I believe these diodes are already rated to 1KV. Probably, the designer added the second diode as a protection against capacitor failure due to one diode shorting. Any one diode can short without blowing up a capacitor.
Voltage rating of capacitor should be greater than 1.25 times peak voltage. (The 1.25 is a safety margin.) Peak voltage is rms voltage * 2^0.5 . In the original circuit, 1.25*220*1.414 = 389. A 400V capacitor is the next larger size. In your case, using a 200V capacitor would be decreasing the safety margin. A 250V cap would be better, but 200V would work.