What is incumbent upon one who doubted the emission of semen after waking up from sleep on the day of his father's death, and performed ablution (wudu') without ghusl, thinking nothing had been emitted, then was later assailed by doubt?

First, masturbation is forbidden and has great dangers and corruptions.

As long as you have not verified the discharge of semen after waking up, ghusl is not obligatory merely due to doubt. Wudu is sufficient for you, and your prayer is valid.

As for your presence during the washing of your father, despite your doubt about being in a state of janabah, there is no sin in it. It is permissible for you to do so even if you were certainly in a state of janabah.