1.  I had used this service while returning back from Kathmandu 2 years ago.
2. You need to request this in advance, otherwise you will not get it. Requesting in airport will not work.
3. I had requested it through Qatar Airways office in Jaya Nepal Cinema Hall in Kathmandu. It took around 3 days to get confirmation.
4. I had bought the ticket through the Qatar Airlines' website directly.
5. After I checked in the airport, they stamped my passport and I was allowed to go out to hotel via a shuttle. Hotel was nice one...

6. Check with Airlines. I do not think it has anything to do how ticket was purchased unless rules are changed.