Has anyone traveled from DC to KTM via. Delta Airlines and Korean Airlines? (US to Seoul - Delta, Seoul to KTM - Korean Airlines)
1. Is one baggage (23 kg) free and checked for Kathmandu (no transfer required by the traveller)?
2. Is hotel provided by the air lines for overnight stay in Korea?
3. Is Visa required for overnight stay in Korea?

This is my experience from Korean airlines DFW- ICN - KTM and LAX - ICN - KTM

1) Two checked in baggage (50 pound) per traveler
2) Hotel is provided one time for round trip ticket, you can choose when to use it.
3) the hotel inside the airport in Seoul is over booked all the time, make sure you book hotel early before travel and let them know when do you need hotel.
4) If you stay in hotel inside airport you do not need visa. They will put you in outside airport hotel if inside hotel has no room.
4) You need visa if you're going outside the airport, and its on arrival visa so no hassle.

by the way did you buy ticket from Korean airlines.

if your transit is more than 12 hours you need to collect your baggage at ICN and check-in again.
They will give you on arrival visa if you are US citizen/GC holder or have US visa on your pasport.

Call delta CS to find more. I had same airlines delta-korean connection from different city and had to cancel at last minute because of baggage transfer policy at ICN