Batel B’shishim doesn’t apply on Pesach and so the comment regarding this confused me. Have I missed something or are you saying it does apply before Pesach commences?
ב"ה Shalom First, allow me to apologize for not answering you sooner. It seems that due to some technical glitch, I was unable to see on my computer that your question had arrived. Although, we are after Pessach there is still what to be learned. Indeed, as you said, Chametz can be Batel B’shishim before Pessach. However, there is a dispute among the Rishonim if chametz which was annulled before Peassch, can it become "chozer Veni'or" on Pessach. Meaning does the annulment become void once Pessach arrives and does the Chametz "arise" once again. The Shulchan Aruch (אורח חיים תמז:ד) says that the chometz does not "chozer Veni'or , meaing we don't view the previous annulment overturned and the chametz remains batel . The Rema, whose opinion the Ashkenazim follow, says that the ruling of the Shulchan that we don't view the previous annulment overturned and the chametz remains batel, only applies to liquids and not solids. So in the case of Ensure since it is liquid what was said in my previous answer applies, especially since the main problem was Kitniyot and not actual Chametz. To sum up, you understood correctly that according to the Shulchan Aruch , Batel B’shishim does apply before Pesach commences. I should point out though, that if a person has a health problem and is need of a particular product, he should rely on only updated Kashrut bulletins. All the best