I agree with you. Since Prof does not know if he succeeded in gifting his power to people, there is a possibility that David is already an Epic, an epic who's capable of taking (or cloning) power from other epics, thus his abilities to dust metals (in disguise with the tensor), to manipulate water (in disguise of spyril), etc. Even David himself does not know about this. When Calamity tried to "gift" power to him, he received the new power anyway. Although Edmund/Conflux commented that epics cannot gift power to other epics, David is unique in this way because he's an epic that takes power from other epics. This "rule" does not apply to him. I think there is one condition that David must do before he could freely use his new found powers, that is, he must PRACTICE.
I believe David might already have attained the power to alter realities but he can't do this yet since he never practiced. So I believe it was all Megan's doing in the end saving David from Prof's deadly forcefield globe.