If there is no speedrun for a game yet, who decides e.g. when the game officially ends (when there is no credit screen)?

The first runner.

I do suggest to start at new game and finish at last screen transition after final input for ease.

To go into a tad bit more detail...the person that submits the run would become the moderator. They would then set rules for the game, presumably based around the run that they had already submitted.