try booking BOM/DEL-KTM in 9W J (with CO to BOM/DEL). Extra bag allowance, and there are some reasonable J fares for the short trip.

overnight layover in DEL is not ideal, but it's not bad in this case (IMO).

It looks like CO flight arrives in DEL 9:20 PM or BOM at 9:40 PM and the 9W flight is at 7:30 AM (from DEL) or 8:05 AM (from BOM).

That's plenty of time to get some rest after a long flight. You'll be able to arrive in KTM more refreshed, IMO (apart from getting used to the jet lag).