Is it the case that certainty is not removed by doubt in the event of self-talk regarding divorce before marriage, knowing that the questioner is of the Shafi'i madhhab?

Our advice is for you to abandon these baseless whisperings (waswas), the cure for which is to forsake them, turn away from them, and ignore them. What you have mentioned does not affect your pursuit of marriage. What you mentioned regarding the Maliki school of thought contradicts their adopted view; for the adopted view is that divorce does not occur by inner speech (kalam nafsi), and those who say it does occur do not mean fleeting thoughts that are not accompanied by intention.