Praise be to Allaah.
We put this question to Shaykh Muhammad ibn ‘Uthaymeen, may Allaah preserve him, who answered as follows:
Praise be to Allaah, Lord of the Worlds, and peace and blessings be upon our Prophet Muhammad, and all his family and companions. Janaazah prayer can be offered for him, so long as he is presumed dead, because we go by what is apparent and most likely to be the case. So his wealth may be inherited if the qaadi rules that he is dead, and his wife should wait out the ‘iddah period, then she is free to remarry – so the janaazah prayer can also be offered for him. And Allaah knows best.