Right, what is needed here is a justification for why the regent came to be, what reason for having a regent take and hold power. And also keep the power.What I am looking for is ideas to make the game run not ideas why not to run the game if that makes sense. So go ahead and poke holes so I can fill them, but don't just keep saying 'its not what happened in real life'
I'm going to set aside the question of the spouse as successor for now.
Now one option for this would be to have the eldest child (the heir) be out of the country (doing some exploring or other such thing), when the father died, though already having had children. In this case the nobles of the nation declare that he is king, however to keep the nation and government going they appoint the now dowager Queen as
"Regent until the the return of the King or her being deemed incapable of holding the office".
Now by the terms of the Regency Charter, dowager Queen is regent, until the King returns - he never does so, however there is also no body recovered and no one knows what happened to him. (This also leave the plot hooks of children of the lost King, real or not to appear). Thus we get to where we are today, with the need to now find an heir or appoint a new "Regent".