Is the statement that the predominance of suspicion (ghalabat al-dhann) takes the place of certainty (yaqeen) when in doubt a matter of scholarly disagreement, and should one base their decision on certainty or on the predominance of suspicion when in doubt?

Acting upon the predominant assumption in matters where certainty cannot be attained is a jurisprudential rule, and the fatwa in question is based on this principle. If you are certain about impurity, then certainty is what should be acted upon. It appears that the questioner suffers from obsessive doubts (waswasah), so she is advised to fear Allah and disregard these whispers. Indeed, religion is easy, and Allah is merciful; He does not burden a soul beyond its capacity.