I would look harder. See if one of the airlines (Indigo flies SIN-CCU, AI flies SIN-DEL) or booking services (Expedia, MakeMyTrip...) will book this trip as a single ticket. If necessary, construct a multi-city itinerary. Google Flights is not an arbiter of what is 'not possible'.
If separate tickets, allow a long time for a connection, maybe 12-24 hours. Even that is not absolutely safe under all circumstances.
PS Delhi is quite a detour.
PPS I don't know if the Inner Line Permit requirement is a complication for booking flights.
Last edited by SeeBuyFly; Aug 31, 2022 at 4:31 pm