Completely agree wid u, I think this can be possible once Sawantwadi Terminus is ready.
We can have 2 sets of trains between Mumbai and Sawantwadi similar to Konkan Kanya and Mandovi with primary maintenance at Sawantwadi just as KK n Mandovi has PM at MAO
Rajya Rani departing Sawantwadi at 20:45 to reach Mumbai at 6:45 can again leave Mumbai as Tutari Express at around 9:00 to reach Sawantwadi at 19:00 in the evening. After overnight maintenance at Sawantwadi, the train will depart in the morning as Tutari express at around 10:00...
more... to reach Mumbai at 20:00 and again departure form Mumbai at 23:55 as Rajya Rani express to arrive Sawantwadi at 9:55. After maintenance at Sawantwadi, the train will head to Mumbai as Rajya Rani at 20:45 n the RSA continues.
Also, since the trains have PM at SWV n are in Mumbai for jus couple of hours, they can start from C.S.T. as well because they wont be occupying the Platforms or sidings fro large amount of time at C.S.T.