Is Nanning airport open 24 hours? Can tourists sleep in the terminal building?

Yes, it is open 24 hours. And you can sleep on the chairs in the terminal building.

Does the airport police allow passengers to sleep at this airport? Very hard to find a room for a foreigner. Most hotels in Nanning only accept Chinese nationals.

Of course foreign passengers are allowed to sleep at the airport. You can have a try to consult whether there is empty room in the airport hotel.

Is it safe to fly round-trip on the same day to the city in January? Is January weather chilly? Will there be rain and snow that should be avoided? Flight arrives in the city by Scoot at 1.50 pm and GX Airlines flight departs for Singapore at 4.05 pm. In airport how many hours before departure much passenger check-in? Unable to find airport hotel listing on Agoda.

Don't worry. It is safe to fly round-trip on the same day to the city in January. And the weather in January would be chilly. A little rain would fall in this month. You may need about 2 hours to finish the transfer in the airport. You can take a rest on the chair in the airport.

I bought GX Airlines one-way ticket to Nanning. From there depart for Phnom Penh via Guangzhou 7 hours later. Travel agent advised at least 3 hours before check-in. Is the airport Terminal 2 big enough for a tourist to sleep?

Of course it is enough for you to rest. Don't worry.