Each of Apex Devastator's four cascade abilities will look for a nonland card with mana value less than 10 (Apex Devastator's mana value). This doesn't change even if one or more of the spells you cast because of those cascade abilities has cascade itself. Each of those additional cascade abilities, if any, will refer to the mana value of the spell that caused it to trigger. In other words, each cascade ability cares only about the spell that caused it to trigger, the stack can get messy, and we wish you luck.