It does not matter if you have an early or late flight, here it is definitely an advantage to be at the airfield early in the morning. Flight times are noise and smoke, in the morning the cards are shuffled. Let your guide help you get on one of the first flights. Even if the weather looks good, everything can change rapidly. If you are on a list, have the security behind you and sitting in the waiting area, it still means keep your fingers crossed. As long as you do not sit on the plane and roll down the runway, everything is open and nothing safe. If the flights have been canceled, you have the option of chartering a helicopter (careful, that can reduce the vacation budget) - provided he can fly. Another (and sometimes the only) option is to stay one more night and hope for better weather the next day. Again, it is an advantage to have a guide. Otherwise, finding an overnight adhoc can quickly turn into another adventure. However, the next day, when the weather is clear, you will not fly to Kathmandu, but to Ramechhap, a small airfield southeast of Kathmandu. From there it is then by bus back to the capital. That's another adventure in itself. In short: You should not plan too soon for the return transfer from the Everest area. Better to take one, better two days buffer.