Quote from RS456
Papa no Iukoto o Kikinasai original manga and anime they are basically dead as in both versions they had a funeral service for them. It is highly unlikely there will be any survivors in a plane crash. Missing can also mean there were bodies but beyond recognition and I am assuming this might be the case since they already had a funeral service for both of them. I am not sure about the novel but in the anime they are definately gone otherwise they wouldn't of gone as far as who is going to adopt who. But you never know the writer might pull a 180 and come up a somewhat ridulous way on how they survived which is also likely since this is also a light hearted manga.
They arent dead in the manga or anime, the funeral service is done because they believe them to be dead since its not likely to survive something like that and felt it as something that was needed to be done, if someone is missing in something like that u dont wait for years to hold off a funeral.