Already there is too much demand on Mumbai-Sawantwadi Route, also with more halts demand would be more, Extending this train to Mangalore would lead to more rush and the very purpose of reducing the crowd wont be served..
No doubt Mangalore deserves a train but it should be altogether a different train departing Mumbai at around 18:00 to reach Mangalore at 10:00.
Few halts can be given in MH n GA as it will reach at odd times during night and more halts in Karnataka.